Exclude Ninja and Xcode from the CMP0021 test
They do not behave the same as the makefile generator with
relative paths.
Don't overwrite the header file for in-source builds.
Unlike other target properties, this does not have a corresponding
non-INTERFACE variant.
This allows propagation of system attribute on include directories
from link dependents.
Eliminate callers of cmMakefile::GetIncludeDirectories.
All callers of GetIncludeDirectories should go through the local generator
object.
Only the local generator calls cmTarget::GetIncludeDirectories directly.