scons: build osmesa swrast and gallium

scons: build osmesa swrast and gallium

This patch makes it possible to build classic osmesa/swrast on windows
again. It was removed in commit 69db422218b0264b5b8eef45bd003a2544e9cbd6.
Although there is a gallium version of osmesa now, the swrast version
still has more features lacking in llvmpipe, e.g. anisotropic filtering.

Tested-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
[Emil Velikov: remove trailing whitespace]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>