[CMake] Multiple makefile builds in a single in-source tree.

[Josef Karthauser]

I'm trying to determine how much work is required modify cmake to
support in-source build trees for multiple Makefile configurations.

Currently, given a source tree such as:

	Source/
		Proj1/
		Proj2/

with CMakeLists.txt files at each level, it is possible to configure a
Makefile style build tree in-source, by use of

	% cd Source
	% cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=RELEASE

for a single configuration type, or out-of-tree using

	% cd Source
	% (mkdir release; cd release; cmake -G 'Unix Makefiles'
-DCMAKE_BUILD_TYPE=RELEASE)
	% (mkdir debug; cd debug; cmake -G 'Unix Makefiles'
-DCMAKE_BUILD_TYPE=DEBUG) 

to create separate file trees configured for each kind of configuration.

What I would like to do is to enable in-source build trees which support
multiple configurations in a single tree. I have a proposal for how it
would work, but I don't know how much of a change to the cmake code it
would take to support it.

The current in-source layout of a build tree looks like:

	Source/
		CMakeLists.txt
		CMakeCache.txt
		CMakeFiles/
			... where the objects are built.
		Makefile

		Proj1/
			CMakeLists.txt
			CMakeFiles/
				...
			Makefile

		Proj2/
			CMakeLists.txt
			CMakeFiles/
				...
			Makefile


What I want to be able to do is to generate 'Makefile' and 'CMakeFiles/'
in a subdirectory of each directory, so if I ran for example:

	% cd Source
	% cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=DEBUG -
DCMAKE_INLINE_BUILD_PREFIX =DEBUGBUILD

it would build the in-line tree configured for DEBUG,

	Source/
		DEBUGBUILD/CMakeLists.txt
		DEBUGBUILD/CMakeCache.txt
		DEBUGBUILD/CMakeFiles/
			... where the objects are built.
		DEBUGBUILD/Makefile

		Proj1/
			DEBUGBUILD/CMakeLists.txt
			DEBUGBUILD/CMakeFiles/
				...
			DEBUGBUILD/Makefile

		Proj2/
			DEBUGBUILD/CMakeLists.txt
			DEBUGBUILD/CMakeFiles/
				...
			DEBUGBUILD/Makefile

Of course the relative paths would be wired so as to allow it to be
built by,

	% cd Source/DEBUGBUILD
	% make

If this was possible, then a number of parallel configuration trees
could be built:

	% cd Source
	% cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=DEBUG
-DCMAKE_INLINE_BUILD_PREFIX=DEBUGBUILD
	% cmake -G 'Unix Makefiles' -DCMAKE_BUILD_TYPE=RELEASE
-DCMAKE_INLINE_BUILD_PREFIX=RELEASEBUILD

generating,

	Source/
		DEBUGBUILD/
		RELEASEBUILD/

		Proj1/
			DEBUGBUILD/
			RELEASEBUILD/

		Proj2/
			DEBUGBUILD/
			RELEASEBUILD/

It would then be a simple step to insert Makefiles in the usual places
that they lived, allowing:

	% cd Source
	% make DEBUG RELEASE
or 
	% make all

or something.


Can someone with a familiarity with the cmake code base point me in the
right direction to implement this please?

Thanks,
Joe