Add support to only build targets under sub-directories.

* Target suffixed by a '/' are considered directory.
* This is a common features provided by build-system based on recursive
  Makefiles.
* This features has been asked on CMake mailing list several times:
  - http://www.cmake.org/pipermail/cmake/2012-February/049263.html
  - http://public.kitware.com/pipermail/cmake-developers/2012-April/003789.html

Some people used to use some wrapper like this to achieve it:

ALL_TARGETS=$(ninja -C ${PROJ_BUILD_DIR-.} -t targets all |
                    grep -v phony | grep "${BUILD_DIR}/" | sed 's/:.*//')

but it was too slow, probably because of all the IOs involve in such a script.
This patch add the same feature internally so it is fast.