Fix the CC_CHECK_FLAG_APPEND macro to test appended flags in context to
current flag values. Otherwise, it is possible to append flags colliding
with user's *FLAGS or even previously appended flags that will cause
the build to fail.
AC_CACHE_CHECK([if $CC supports flag $3 in envvar $2],
AS_TR_SH([cc_cv_$2_$3]),
[eval "AS_TR_SH([cc_save_$2])='${$2}'"
- eval "AS_TR_SH([$2])='-Werror `echo "$3" | sed 's/^-Wno-/-W/'`'"
+ eval "AS_TR_SH([$2])='${cc_save_$2} -Werror `echo "$3" | sed 's/^-Wno-/-W/'`'"
AC_LINK_IFELSE([AC_LANG_SOURCE(ifelse([$4], [],
[int main(void) { return 0; } ],
[$4]))],