pkg-config: add vendored pc files to default path

Just rebottling, but here's an explanation:

Homebrew provides some missing pc files for recent OS X versions, and they should be available to pkg-config by default.

Note that under both stdenv and superenv, we override the built-in path, so changes to the formula do not affect the stability of builds.

For example (#21171), `hwloc` uses the system libxml2, yet:

```
$ pkg-config hwloc --cflags
Package libxml-2.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libxml-2.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libxml-2.0', required by 'hwloc', not found
```

We provide a libxml2 pc file during the build, so it should be available at runtime as well.