regex: port to mingw's recent addition of undeclared alarm

regex: port to mingw's recent addition of undeclared alarm

On mingw (at least, when cross-compiling with Fedora 18's
mingw32-headers-2.0.999-0.15.trunk.20121110.fc18.noarch build),
compilation of test-regex fails:

test-regex.c: In function 'main':
test-regex.c:42:11: error: 'SIGALRM' undeclared (first use in this function)
test-regex.c:42:11: note: each undeclared identifier is reported only
once for each function it appears in
test-regex.c:43:3: warning: implicit declaration of function 'alarm'

It turns out that recent mingw64 added an export of alarm() and
SIGALRM, but guarded their declarations behind __USE_MINGW_ALARM
(default off, and with alarm() only in the non-standard <io.h>);
so the m4 tests were setting HAVE_ALARM to 1 based on link success
but then failing to compile.

* doc/posix-functions/alarm.texi (alarm): Document that alarm
exists but still doesn't work in newer mingw.
* m4/frexp.m4 (gl_FUNC_FREXP_WORKS): Check for alarm declaration,
not existence.  Ensure SIGALRM is not trapped.
* m4/mktime.m4 (gl_FUNC_MKTIME): Likewise.
* m4/regex.m4 (gl_REGEX): Likewise.
* m4/remainderf.m4 (gl_FUNC_REMAINDERF_WORKS): Likewise.
* tests/test-regex.c (main): Use correct probe for alarm.

Signed-off-by: Eric Blake <eblake@redhat.com>