Defining _GNU_SOURCE gives us native asprintf() and strptime() on glibc

systems, and has no effect on FreeBSD.

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@768 d4fa192b-c00b-0410-8231-f00ffab90ce4

configure.ac

@@ -27,6 +27,7 @@ AC_ARG_ENABLE(werror,
 	CFLAGS="${CFLAGS} -Werror")
 
 # Checks for programs.
+AC_GNU_SOURCE
 AC_PROG_CC
 AC_PROG_CPP
 AC_PROG_INSTALL
@@ -61,6 +62,7 @@ AC_FUNC_VPRINTF
 AC_CHECK_FUNCS([strerror])
 AC_FUNC_STRERROR_R
 AC_CHECK_FUNCS([socket])
+AC_CHECK_FUNCS([strptime])
 
 # These functions are provided by libcompat on platforms where they
 # are not available