contrib: fix PKG_CONFIG_PATH environment variable

main.mak adds local pkg-config paths to the env. var. PKG_CONFIG_PATH
after the existing configuration, so local are tested after globals.
This is an issue if globals contains a package outdated for our needs.
The local setting must be first.

Change-Id: I8f9e060f05d4ac662ac5c23b0dbeaea3b67bd401
Tuleap: #783

contrib/src/main.mak

@@ -65,7 +65,7 @@ else
 PKG_CONFIG ?= pkg-config
 endif
 
-PKG_CONFIG_PATH := $(PKG_CONFIG_PATH):$(PREFIX)/lib/pkgconfig:$(PREFIX)/lib/$(HOST)/pkgconfig
+PKG_CONFIG_PATH := $(PREFIX)/lib/pkgconfig:$(PREFIX)/lib/$(HOST)/pkgconfig:$(PKG_CONFIG_PATH)
 export PKG_CONFIG_PATH
 
 ifdef HAVE_CROSS_COMPILE