Also applies to *_BINARY_DIR.

This effectively reverts 84083dcc,
which broke all users of libgit2 that use it as a CMake subdirectory
(via `add_subdirectory()`). This is because CMAKE_SOURCE_DIR refers
to the root-most CMake directory, which in the case of
`add_subdirectory()` is a parent project to libgit2 and thus the paths
don't make any sense to the configuration files. Corollary,
CMAKE_SOURCE_DIR only makes sense if the CMake project is always the
root project - which can rarely be guaranteed.

In all honesty, CMake should deprecate and eventually remove
CMAKE_SOURCE_DIR and CMAKE_BINARY_DIR. It's been the source of headaches
and confusion for years, they're rarely useful over
CMAKE_CURRENT_(SOURCE|BINARY)_DIR or PROJECT_(SOURCE|BINARY)_DIR,
and they cause a lot of confusing configuration and source
code layouts to boot.

Any time they are used, they break `add_subdirectory()` almost 100% of
the time, cause confusing error messages, and hide subtle bugs.