Slight improvements to our Windows batch files [ci skip].
Some minor cleaning up [ci skip].
Some work towards supporting building OpenCOR with Ninja (#364).
Some minor cleaning up [ci skip].
Some minor cleaning up [ci skip].
Revert "Some minor cleaning up [ci skip]."

This reverts commit b66ae7e0bbf2faf6c22d26e0607084467e048c8a.
Some work towards supporting building OpenCOR with Ninja (#364) [ci skip].
Some minor cleaning up [ci skip].
Some work towards supporting building OpenCOR with Ninja (#364) [ci skip].

Not only does this work with Ninja (on Linux and OS X since it was
already working fine on Windows), but it’s also neater.
Done with supporting building OpenCOR with Ninja (closes #364).
Done with supporting building OpenCOR with Ninja (closes #364).
Done with supporting building OpenCOR with Ninja (closes #364).

I thought our combined use of ADD_CUSTOM_COMMAND() and
ADD_CUSTOM_TARGET() to add a plugin binary would work on all three
platforms, but it only works on OS X! So, going for EXECUTE_PROCESS()
instead. In the end, it’s an acceptable solution since this kind of
files doesn’t have dependencies.
Slight update to the developer documentation, now that we support building with Ninja [ci skip].
Slight update to the LLVM README.txt file since we now support building OpenCOR with ninja [ci skip].
Some work towards upgrading LLVM from version 3.4.1 to version 3.4.2 (#372) [ci skip].
Some minor cleaning up [ci skip].
Done upgrading LLVM from version 3.4.1 to version 3.4.2 (closes #372).
Done upgrading LLVM from version 3.4.1 to version 3.4.2 (closes #372).
Some minor cleaning up [ci skip].
Fixed a small issue with our run[tests].bat files [ci skip].