gcc does not support testing for negated warnings. See here for details:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63499
This patch changes CC_CHECK_FLAG_APPEND to always test for the non-negated
warnings.
All tests for compiler support were failing with:
/usr/bin/ld: /tmp/ccwnVc2A.o: relocation R_ARM_MOVW_ABS_NC against `a' can not be used when making a shared object; recompile with -fPIC
Use AC_LINK_IFELSE instead of AC_COMPILE_IFELSE to test for flags that
might succeed during compilation but not during linking. An example is gcc
compiled with libssp support but gnu-ld without it. In this case
-fstack-protector works fine during compilation but fails during linking
as several internal helpers are missing.
Use _builtin_uaddll_overflow/_builtin_uaddl_overflow when available,
abstracting the type to use it with uint64_t.
Otherwise fallback to the implementation as added in 67466f2 ("Prevent
offset + size overflow.").
This also adds the tests for this new helper in the testsuite.
In order to let us use "#if HAVE__BUILTIN_*" without checking if it's
actually define, make sure we define it to 0 in config.h when the
function is not available.
Clang doesn't treat unknown warnings flags as an error, but rather as a
warning. The result is that the detection for whic CFLAGS are supported
by this compiler will not work, since the compilation will succeed.
With this patch we now successfully detect clang doesn't support
-Wlogical-op, as opposed to previous behavior:
checking if clang supports flag -Wlogical-op in envvar CFLAGS... no
We use this macro only for LDFLAGS and CFLAGS, so it's safe to stash
-Werror there.
These variables are supposed to be set by user. What we can do in
configure is to set another variable and AC_SUBST() it. Then in
Makefile.am we assign it to AM_{CFLAGS,LDFLAGS}. This way user can
always override their values, in configure or make phase.
Reference: http://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html
This is mostly just preparation for the next patch. But this macro is
used for flags that are only needed during linking but that don't
make sense for normal compilation.
I saw this exact type of patch online for systemd, but it never seems to
have actually been incorporated into their attributes.m4.
