configure: fix the PKG_CONFIG_PATH when building on mingw32 shells

The mingw32/mingw64 pkg-config doesn't handle UNIX pathes

configure.ac

@@ -467,7 +467,15 @@ AS_IF([test -n "${CONTRIB_DIR}"], [
   CFLAGS="${CFLAGS} -I${CONTRIB_DIR}/include"
   CXXFLAGS="${CXXFLAGS} -I${CONTRIB_DIR}/include"
   OBJCFLAGS="${OBJCFLAGS} -I${CONTRIB_DIR}/include -F${CONTRIB_DIR} -F${CONTRIB_DIR}/Frameworks"
-  export PKG_CONFIG_PATH="${CONTRIB_DIR}/lib/pkgconfig:${CONTRIB_DIR}/share/pkgconfig:$PKG_CONFIG_PATH"
+  PKG_CONFIG_PATH="${CONTRIB_DIR}/lib/pkgconfig:${CONTRIB_DIR}/share/pkgconfig:${PKG_CONFIG_PATH}"
+  case "${build_os}" in
+      mingw32)
+          export PKG_CONFIG_PATH=`cygpath -mp ${PKG_CONFIG_PATH}`
+      ;;
+      *)
+          export PKG_CONFIG_PATH="${PKG_CONFIG_PATH}"
+      ;;
+  esac
   LDFLAGS="${LDFLAGS} -L${CONTRIB_DIR}/lib"
 
   AS_IF([test -z "$LUAC"], [