[PATCH 2/3] configure.ac: Fix detecting sighandler_t on glibc

[Sam James]

Fixes the following error/misdetection of sighandler_t:
```
config.log:2639:conftest.c:83:13: error: 'sighandler_t' undeclared (first use 
in this function); did you mean 'sa_handler'?
```

sighandler_t requires -D_GNU_SOURCE on glibc systems.

Signed-off-by: Sam James <sam@gentoo.org>
---
 configure.ac | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 2b505c783..c2276c7d9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -490,7 +490,8 @@ AC_CHECK_FUNCS([cbrt])
 AC_CHECK_FUNCS([getmsg putmsg])
 
 dnl sighandler_t apparently not defined in Apple/OS X
-AC_CHECK_TYPES([sighandler_t], [], [], [[#include <signal.h>]])
+AC_CHECK_TYPES([sighandler_t], [], [], [[#define _GNU_SOURCE
+#include <signal.h>]])
 
 # At least on freebsd, putc_unlocked is a macro, so the standard
 # AC_CHECK_FUNCS doesn't work well.
-- 
2.38.1