Added new var "build_wrksrc" for use in templates.

This var sets a relative path to $wrksrc that is used to
configure/build/install a package.

--HG--
extra : convert_revision : 690f4d435f3d4e6a9228f7f5f09ecf8b490a273a

xbps-src/shutils/build_funcs.sh

@@ -43,7 +43,8 @@ build_src_phase()
 
 	[ ! -d $wrksrc ] && msg_error "unexistent build directory [$wrksrc]"
 
-	cd $wrksrc || exit 1
+	cd $wrksrc || return 1
+	[ -n "$build_wrksrc" ] && cd $build_wrksrc || return 1
 
 	[ -n "$revision" ] && pkg="${pkg}_${revision}"