libfreenect: backporting upstream patch to stop clobbering CFLAGS

Backporting following libfreenect patch:
https://github.com/OpenKinect/libfreenect/commit/
0cbb86ec982d3e79ad5efa9d1b2b16724ae89bc5

Without this patch, libfreenect would override all compiler flags,
including any machine specific compile flags
(e.g. -mfloat-abi=hard to compile with hard floating point).

We are backporting this patch rather than upgrading to v0.4.1
where this is fixed, as v0.4.0 introduced changes that break
compatibility with the current freenect version (0.3.2).
See https://github.com/OpenKinect/libfreenect/releases/tag/v0.4.0
for the breaking changes.
See https://github.com/ros-drivers/freenect_stack/issues/12
for additional discussion.