Detect: Factor out check for compiler builtin headers into helper function
Detect: Factor out check for compiler builtin headers into helper function

Since commit 066090f9 (Detect: Fix GNU builtin include directory
detection on ppc64, 2019-07-15, v0.2.1~3^2~1) the check is complex
enough to deserve a dedicated helper.

bradking/CastXML,ellert/CastXML,bradking/CastXML,CastXML/CastXML,CastXML/CastXML,CastXML/CastXML,ellert/CastXML,bradking/CastXML,ellert/CastXML