CMake used to put all header search paths into HEADER_SEARCH_PATHS
attribute. Unfortunately this attribute does not support to declare
a search path as a system include.
As a hack one could add a -isystem /path to the cflags but then include
ordering is not deterministic. A better approach was chosen with this
patch by not filling HEADER_SEARCH_PATHS at all and to populate
the C, C++, and Fortran flags directly. The include paths used by
Xcode should be now identical to the ones used by Unix Makefiles and
Ninja generator.
Allow the `Swift` language to be enabled with the Xcode generator for
Xcode >= 6.1. Reject it on other generators and with older Xcode
versions. Since Apple is the only vendor implementing the language
right now, the compiler id can be just `Apple`.
Gregor Jasny
Brad King