Don't pick up libexecinfo (devel/libexecinfo) if it's installed.

--- configure.orig	Sat Apr 28 22:57:00 2007
+++ configure	Mon Oct 27 18:05:17 2008
@@ -7445,7 +7445,7 @@ ac_res=`eval echo '${'$as_ac_Header'}'`
 echo "${ECHO_T}$ac_res" >&6; }
 
 fi
-if test `eval echo '${'$as_ac_Header'}'` = yes; then
+if false test `eval echo '${'$as_ac_Header'}'` = yes; then
   cat >>confdefs.h <<_ACEOF
 #define `echo "HAVE_$ac_header" | $as_tr_cpp` 1
 _ACEOF
