Sebastian Holtermann authored Mar 28, 2019 and Brad King committed Mar 28, 2019

The `cmLocalGenerator::GetIncludeDirectoriesImplicit` method is called
by `cmQtAutoGenInitializer` to get the compiler's list of implicit
include directories.  Since commit 557b2d6e (Fix regression in
-I/usr/include exclusion logic, 2019-02-13, v3.14.0-rc2~6^2~2) the
method hard-codes exclusion of `/usr/include` for historical reasons.
However, it should not be reported as a real implicit include directory
unless the compiler really has it.

Refactor the logic to distinguish the hard-coded exclusion of
`/usr/include` from the real list of implicit include directories.

Fixes: #19100