thunderbird: fix cross-build.

srcpkgs/thunderbird/template

@@ -1,7 +1,7 @@
 # Template build file for 'thunderbird'.
 pkgname=thunderbird
 version=38.3.0
-revision=3
+revision=4
 wrksrc="comm-esr${version/.*/}"
 short_desc="Standalone Mail/News reader"
 maintainer="Enno Boland <gottox@voidlinux.eu>"
@@ -94,6 +94,9 @@ do_configure() {
 		--enable-optimize="$CFLAGS" --disable-strip --disable-install-strip \
 		--disable-static --enable-pie \
 		--disable-profiling --disable-profilelocking
+	if [ "$CROSS_BUILD" ]; then
+		sed -i "s#-L/usr/lib ##" `find . -name '*.mk' -or -name Makefile`
+	fi
 }
 do_build() {
 	cd xbps-build