efl: use vanilla pkg-config

Use vanilla pkg-config to build EFL applications.

The EFL library has a set of pkg-config files (*.pc) which uses
private requirements. The default pkg-config setup on nixpkgs is
patched to disable resolving those requirements. See
http://bugs.freedesktop.org/show_bug.cgi?id=4738 for reference.

As a consequence each package depending on efl has to explicitly set
the search path in order to be able to find the corresponding header
files.

By using vanilla pkg-config this is not necessary (and this is the
expected behaviour for pkg-config), allowing simpler nix expressions.