Only pass INTERNAL_INCLUDE_DIRS to header file language check.

We don't care about headers in external include dirs potentially
including our local headers - that is never going to happen, they
are never going to be scanned and it takes an extraordinary amount
of resources to scan all of them (eg, boost)
Merge pull request #7 from smspillaz/include-what-you-use-target-cmake.item_7

Only pass INTERNAL_INCLUDE_DIRS to header file language check.