Merge pull request #51 from bchretien/master

pkg-config.cmake: use CACHE INTERNAL for _PKG_CONFIG_* variables.
Fix CMP0007 policy problem + .catkin removal in case of catkin package.
Merge pull request #52 from jrl-umi3218/topic/catkin-uninstall

Fix CMP0007 policy problem + .catkin removal in case of catkin package.
python.cmake: add a PYTHON_BUILD macro.

This macros provides a way to build some source file and store them in
the build directory. Then, the install macro can copy those files to the
adequate destination.

Thus, the compilation can be done without calling the "install" target,
which makes it easier to test local modules. We rely on
ADD_CUSTOM_TARGET for the compilation of each file.