libmill: don't set _FORTIFY_SOURCE

libmill does quite a bit of stack trickery that triggers assertions when
build with our default _FORTIFY_SOURCE=2

srcpkgs/libmill/template

@@ -1,7 +1,7 @@
 # Template file for 'libmill'
 pkgname=libmill
 version=1.6
-revision=1
+revision=2
 build_style=gnu-configure
 hostmakedepends="automake libtool"
 short_desc="Go-style concurrency in C"
@@ -15,6 +15,11 @@ pre_configure() {
 	autoreconf -fi
 }
 
+do_configure() {
+	export CFLAGS="${CFLAGS/-D_FORTIFY_SOURCE=?/}"
+	${configure_script} ${configure_args}
+}
+
 post_install() {
 	vlicense COPYING
 }