[llvm-dev] Guidance on cross compiling LLVM with mingw-w64 and cmake

[Tony Kelman via llvm-dev]

I need to build libLLVM (individual static libraries are fine at the
moment) using mingw-w64 cross compilers, i686-w64-mingw32-gcc and
(separately) x86_64-w64-mingw32-gcc. I'd like this to work from both
Linux and Cygwin build environments. With autotools, this worked fine:
../configure --host=i686-w64-mingw32 and that's it (with mingw32-gcc-c++
installed on Fedora 23, also works fine on Ubuntu, Cygwin, etc).
I'm trying to recreate this with cmake so we don't get stuck on 3.8
indefinitely. Here's what I've got so far:

cmake .. -DCMAKE_C_COMPILER=/usr/bin/i686-w64-mingw32-gcc \
  -DCMAKE_CXX_COMPILER=/usr/bin/i686-w64-mingw32-g++ \
  -DCMAKE_SYSTEM_NAME=Windows \
  -DCMAKE_RC_COMPILER=/usr/bin/i686-w64-mingw32-windres
# (some older versions of cmake have issues if you don't
# specify an absolute path to windres)

When this gets to "Configuring NATIVE targets" it calls cmake again
trying to use the same mingw compilers, but without CMAKE_SYSTEM_NAME
set so it doesn't succeed in configuring. During the build I then get
a "No rule to make target 'LLVMSupport'.  Stop." Full build log is at
http://sprunge.us/bCUU

I'd like to go with the default behavior of building a native version
of TableGen, but the way that's currently set up in cmake isn't working
here. I'm looking at cmake/modules/CrossCompile.cmake (and I think
"toochain" on the first line is a typo?) and trying to figure out how
to make its call to execute_process(COMMAND ${CMAKE_COMMAND} ...) not
inherit the top-level settings for CMAKE_C_COMPILER etc from the cross
build. Any ideas? I've tried moving my mingw settings from the command
line to a toolchain file but that hasn't done anything different so far.
I've also tried specifically creating a native toolchain file and
tweaking the call to llvm_create_cross_target_internal at the end of
CrossCompile.cmake to use it, but that hasn't worked either - keeps giving
Could not find toolchain file: "/home/llvm/cmake/platforms/NATIVE.cmake"

Is this a configuration anyone else has gotten working with cmake?
Thanks,
Tony