AC_USE_SYSTEM_EXTENSIONS will modify CFLAGS if nothing was in there

beforehand.  We don't want that.  So if there was nothing in
CFLAGS, put nothing back in there.

This commit was SVN r23182.

configure.ac

@@ -65,7 +65,14 @@ AS_IF([test "$host" != "$target"],
       [AC_MSG_WARN([Cross-compile detected])
        AC_MSG_WARN([Cross-compiling is only partially supported])
        AC_MSG_WARN([Proceed at your own risk!])])
+# AC_USE_SYSTEM_EXTENSIONS alters CFLAGS (e.g., adds -g -O2)
+CFLAGS_save=$CFLAGS
 AC_USE_SYSTEM_EXTENSIONS
+# AC_USE_SYSTEM_EXTENSIONS will modify CFLAGS if nothing was in there
+# beforehand.  We don't want that.  So if there was nothing in
+# CFLAGS, put nothing back in there.
+AS_IF([test -z "$CFLAGS_save"], [CFLAGS=$CFLAGS_save])
+CFLAGS_save=
 
 # Get our platform support file.  This has to be done very, very early
 # because it twiddles random bits of autoconf