firefox: set rpath to make this work ootb with musl.

srcpkgs/firefox/template

@@ -1,7 +1,7 @@
 # Template build file for 'firefox'.
 pkgname=firefox
 version=38.0.6
-revision=1
+revision=3
 wrksrc="mozilla-release"
 short_desc="Lightweight gecko-based web browser"
 maintainer="Juan RP <xtraeme@voidlinux.eu>"
@@ -64,6 +64,9 @@ do_configure() {
 		;;
 	esac
 
+	mkdir -p /usr/lib/firefox
+	export LDFLAGS+=" -Wl,-rpath=/usr/lib/firefox"
+
 	cd xbps-build
 	SHELL=/bin/bash ../configure --prefix=/usr --libdir=/usr/lib \
 		--with-system-nspr --with-system-nss --with-system-bz2  \