Set a POSIX level in 1 source, rather than GNU extensions for everything.

To reduce risks concerning portability and compatibility, and to
avoid accidentally depending on non-standard features.

configure.ac

@@ -14,7 +14,6 @@ AM_PROG_AR
 LT_PREREQ([2.2.6])
 LT_INIT([dlopen disable-static])
 
-AC_GNU_SOURCE
 AC_PROG_CC
 AC_PROG_CC_STDC
 if test "x$ac_cv_prog_cc_c99" = xno; then

src/vmod_pesi.c

@@ -27,6 +27,9 @@
  * SUCH DAMAGE.
  */
 
+/* for pthread_rwlock_t from cache/cache_vcl.h */
+#define _POSIX_C_SOURCE 200809L
+
 #include "config.h"
 
 #include "cache/cache_varnishd.h"