sato-icon-theme: simplify icon-name-mapping lookup

sato-icon-theme: simplify icon-name-mapping lookup

Patching an option into configure.ac and then running sed over path variables is
fragile as the definition of $libexecdir can and has changed.

Instead, tell configure to use our pkg-config-native wrapper when it uses
pkg-config, and it will find the right binaries without any further munging.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>