Cmake Crash Course
Cmake builds makefiles in the current directory. It leaves a cache file called CMakeCache.txt, even when cmake fails. Always delete this, as command line options don't override what's in there. It's common to bang your head on cmake only to remember that the cache file contains your first mistake.
There are two intermixed cmake instances in reactos, one for host tools and one for reactos itself. ReactOS is built by specifying an alternate toolchain definition for reasons nobody has explained to me. The one for unixen is toolchain-mingw32.cmake and you tell cmake to use it with -DCMAKE_TOOLCHAIN_FILE= .. It'd be nice to name that variable something more understandable.
make a tools dir somewhere (like ./build/tools)
cmake -G "Unix Makefiles" [reactos-dir] TOOLS=`pwd`
make a different directory (like ./build/ros).
NOTE: This directory will contain a different logical cmake instance.
cmake -G "Unix Makefiles" -DREACTOS_BUILD_TOOLS_DIR:DIR=$TOOLS -DREACTOS_TOOLCHAIN_FILE=toolchain-mingw32.cmake
NOTE: The path to the tools dir *must* be specified as a full path
When cmake fails, errors look like this:
CMake Error at CMakeLists.txt:90 (include): include could not find load file:
The most common reason for this step to fail is not specifying the path to the tools.