perl: add -D_GNU_SOURCE on musl to find correct prototype for memmem

Also enable -Wall so things like this can be detected earlier.

srcpkgs/perl/template

@@ -1,7 +1,7 @@
 # Template build file for 'perl'.
 pkgname=perl
 version=5.26.0
-revision=1
+revision=2
 _perl_cross_version=1.1.6
 build_style=gnu-configure
 hostmakedepends="less"
@@ -169,13 +169,15 @@ do_configure() {
 	# (default on musl.)
 	case "$XBPS_TARGET_MACHINE" in
 	i686|armv6l|armv7l)
-		CFLAGS+=" -D_FILE_OFFSET_BITS=64 -DLARGE_FILE_SUPPORT "
+		CFLAGS+=" -D_FILE_OFFSET_BITS=64 -DLARGE_FILE_SUPPORT ";;
+	*-musl)
+		CFLAGS+=" -D_GNU_SOURCE";;
 	esac
 
 	export LD="$CC"
 	./configure --prefix=/usr $_args \
 		-Dcccdlflags="-fPIC" \
-		-Doptimize=" ${CFLAGS} " -Dccflags=" ${CFLAGS} " \
+		-Doptimize=" -Wall ${CFLAGS} " -Dccflags=" ${CFLAGS} " \
 		-Dlddlflags="-shared ${LDFLAGS}" -Dldflags="${LDFLAGS}" \
 		-Dperl_static_inline='static __inline__' -Dd_static_inline ||
 	{