Merge pull request #1 from aabadie/superbuild

Superbuild using cmake externalProject instead of subdirectories.
removed submodules.
Simplified the script, made the proper interface for the setting of CMAKE_BUILD_TYPE, added options to avoid the build of some libraries (using system's versions instead (not yet fully implemented).
Add the possibility of overriding external projects by putting links in the superbuild directory.
Merge branch 'papadop-master' into master-merge

Conflicts:
	CMakeLists.txt