cmake: add support for "out-of-source" build

[xaver42]

Cmake has the ability to perform a build in a dedicated build directory without modifying the original source directory:
http://www.cmake.org/Wiki/CMake_FAQ#Out-of-source_build_trees

Some software projects don't support in-source builds, so to avoid hacks in the easyconfig recepies it would be good to add an option in cmakemake.py that can be set from the easyconfig to request an "out-of-source" build.

[JensTimmerman]

Well, we already have a 'build in install dir' option, which would probably work, but you are right, we should add this options, it should simply create a build dir, cd into it, and run cmake .. (we could even do this by default all the time)

[fgeorgatos]

as of 7/11/2013:

[5:27pm] wpoely: boegel: with cmake it's custom to not build in the src dir
[5:27pm] wpoely: but the CMakeMake does this by default
[…]
[5:30pm] jgtimmer: feel free to change, also we should set the cmake_include_path, and the cmake_library_path and the cmake_program_path to what we have in CFLAGS,  LD_LIBRARY_PATH and PATH respectively
[5:31pm] jgtimmer: then we can remove all the -DBOOST_PATH=$BROOTBOOST stuff
[5:31pm] jgtimmer: because Cmake is supposed to find it then

[wpoely86]

Added in #303