gcc (AC_CHECK_HEADERS) is using $CPP to check if the header pass
the precompiler, if compiling with $CC failed.
Sadly $CC is xgcc of gcc, and $CPP is 'gcc -E'. And as you build
on a glibc machine, precompiling of the headers (execinfo.h and
iconv.h will succeed). As a dirty hack we will handle this case
as failure.
This fixes the libmudflap build for uclibc which lacks execinfo.h
but has HAVE_EXECINFO_H defined

Index: gcc-4.0.2/libmudflap/configure
===================================================================
--- gcc-4.0.2.orig/libmudflap/configure	2006-04-30 16:57:47.000000000 +0200
+++ gcc-4.0.2/libmudflap/configure	2006-04-30 17:00:59.000000000 +0200
@@ -3719,6 +3719,8 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    # hack, hack, hack
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: $ac_header: present but cannot be compiled" >&5
 echo "$as_me: WARNING: $ac_header: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: $ac_header:     check for missing prerequisite headers?" >&5
@@ -3731,6 +3733,8 @@
 echo "$as_me: WARNING: $ac_header: proceeding with the preprocessor's result" >&2;}
     { echo "$as_me:$LINENO: WARNING: $ac_header: in the future, the compiler will take precedence" >&5
 echo "$as_me: WARNING: $ac_header: in the future, the compiler will take precedence" >&2;}
+    { echo "$as_me:$LINENO: WARNING: $ac_header: Zecke says do not trust the wrong cpp! Ignoring." >&5
+echo "$as_me: WARNING: $ac_header: Zecke says do not trust the wrong cpp! Ignoring." >&2;}
     (
       cat <<\_ASBOX
 ## ------------------------------------- ##
Index: gcc-4.0.2/libstdc++-v3/configure
===================================================================
--- gcc-4.0.2.orig/libstdc++-v3/configure	2006-04-30 16:57:53.000000000 +0200
+++ gcc-4.0.2/libstdc++-v3/configure	2006-04-30 22:59:00.000000000 +0200
@@ -30513,6 +30513,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -30525,6 +30526,8 @@
 echo "$as_me: WARNING: iconv.h: proceeding with the preprocessor's result" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h: in the future, the compiler will take precedence" >&5
 echo "$as_me: WARNING: iconv.h: in the future, the compiler will take precedence" >&2;}
+    { echo "$as_me:$LINENO: WARNING: $ac_header: Zecke says do not trust the wrong cpp! Ignoring." >&5
+echo "$as_me: WARNING: $ac_header: Zecke says do not trust the wrong cpp! Ignoring." >&2;}
     (
       cat <<\_ASBOX
 ## ----------------------------------------- ##
@@ -53622,6 +53625,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -75885,6 +75889,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -78600,6 +78605,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -80969,6 +80975,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -83214,6 +83221,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -86227,6 +86235,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -88425,6 +88434,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -90672,6 +90682,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -93280,6 +93291,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -94996,6 +95008,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5
@@ -97337,6 +97350,7 @@
     ac_header_preproc=yes
     ;;
   no:yes:* )
+    ac_header_preproc=no
     { echo "$as_me:$LINENO: WARNING: iconv.h: present but cannot be compiled" >&5
 echo "$as_me: WARNING: iconv.h: present but cannot be compiled" >&2;}
     { echo "$as_me:$LINENO: WARNING: iconv.h:     check for missing prerequisite headers?" >&5