scons: Include headers from the local include directory first.

scons: Include headers from the local include directory first.

Certain headers, such as GL/glew.h, are in both the Mesa include and the
default installed include directories. On recent distros the needed
symbols can be found in both places. On older distros the installed
headers could be lacking symbols, so for a header that exists in both
places, the local one should be found first.