flex: fix for glibc-2.26

We need to define _GNU_SOURCE to make stdlib.h define the function
reallocarray(). If not defined, flex builds with a warning and
crashes on invocation.

srcpkgs/flex/template

@@ -1,7 +1,7 @@
 # Template file for 'flex'
 pkgname=flex
 version=2.6.4
-revision=1
+revision=2
 build_style=gnu-configure
 configure_args="--disable-bootstrap --disable-shared"
 hostmakedepends="bison"
@@ -14,6 +14,9 @@ license="BSD"
 distfiles="https://github.com/westes/${pkgname}/releases/download/v${version}/${pkgname}-${version}.tar.gz"
 checksum=e87aae032bf07c26f85ac0ed3250998c37621d95f8bd748b31f15b33c45ee995
 
+# Required to enable the definition of reallocarray() in stdlib.h
+CFLAGS="-D_GNU_SOURCE"
+
 post_install() {
 	vlicense COPYING
 }