Don't override CC with cmake and CROSS_TOOLCHAIN

[brooks]

Summary

The current native (non-qemu-user-mode) cross build support works by
overriding CC, CXX, etc with versions that build target binaries. This
doesn't work with CMake because it expects that those build native
binaries and that a toolchain file be supplied to build target binaries.

This change includes the toolchain file to get XCC, etc, but doesn't
override CC when USES contains cmake.

[brooks]

With this I've been able to construct a set of changes to llvm-devel to allow it to cross build a dependency-free version of clang which is most of the way to having base/llvm* ports suitable for use with a root fs containing no compiler.

[brooks]

ping

[jhb]

This seems fine to me in terms of how it effects using CROSS_TOOLCHAIN for base/*, but I can't speak to whether or not checking USES directly is the best way to check for cmake.

[bapt]

Doing this we might miss cases where the cmake uses is inherited from a USES (which does not exist in the ports tree - yet ? but is a valid usage)

[brooks]

In D29236#669332, @bapt wrote:

Doing this we might miss cases where the cmake uses is inherited from a USES (which does not exist in the ports tree - yet ? but is a valid usage)

Hmm, an alternative might be do gate this on defined(GNU_CONFIGURE) instead, but I think that also suffers from the implied definition issue since this is so early in bsd.port.mk (actual code doesn't start until after line 1000).

[brooks]

We could also do something gross like adding an && !defined(DONT_OVERRIDE_TOOLCHAIN_FOR_CROSS_TOOLCHAIN). At the moment, I just want something that works since the current code makes it impossible to cross build cmake ports.