Update path to pkg-config header file.

[Kerilk]

This is the accompanying pull request for KhronosGroup/OpenCL-Headers#218

The first commit refactors the way the CI handles PKG_CONFIG_PATH.
The second commit updates the path to the newly adopted path in the Headers (failure is expected until the Headers are merged).

[bashbaug]

This looks good to me.

To merge this should we:

• First merge the OpenCL headers changes.
• Trigger a CI build.
• Merge the changes assuming all tests pass?

[Kerilk]

This looks good to me.

To merge this should we:

* First merge the OpenCL headers changes.

* Trigger a CI build.

* Merge the changes assuming all tests pass?

Yes this is the plan. I will rerun the failed tests once the headers changes is through.

[bashbaug]

Looks like everything is passing - awesome. Merging this one too...