cmake: Replace pkg_check_modules with waffle_pkg_config

Define a new macro waffle_pkg_config, which replaces each occurence of
pkg_check_modules.

Macro waffle_pkg_config is a smart wrapper around CMake's standard
pkg_check_modules macro.

waffle_pkg_config fixes a fatal flaw of pkg_check_modules.  Unlike
pkg_check_modules, waffle_pkg_config calls pkg-config and checks for the
requested package *each* time the macro is called.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>