Rename psq_strip_add_custom_target_sources to psq_strip_extraneous_sources

The new function will strip out all non-source and non-header files,
which generally trip up the tools.
Merge pull request #12 from smspillaz/tooling-cmake-util.item_12

Rename psq_strip_add_custom_target_sources to psq_strip_extraneous_sources