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.
