FindPkgConfig: add PREFIX/share/pkgconfig to PKG_CONFIG_PATH (#15910)

Most packages install their .pc files into PREFIX/lib/pkgconfig, but
some put them into PREFIX/share/pkgconfig. Either is valid, and
pkg-config itself looks in both for the /usr and /usr/local prefixes.

This fixes an issue where some packages (yajl, for example) are not
found if they are installed into a non-standard prefix and
CMAKE_PREFIX_PATH is used to locate them.

Modules/FindPkgConfig.cmake

@@ -263,6 +263,7 @@ macro(_pkg_check_modules_internal _is_required _is_silent _no_cmake_path _no_cma
         endif()
       endif()
       list(APPEND _lib_dirs "lib/pkgconfig")
+      list(APPEND _lib_dirs "share/pkgconfig")
 
       # Check if directories exist and eventually append them to the
       # pkgconfig path list