Read MESA_VERSION once

There is no need to re-read MESA_VERSION every time, and it is showing
up at the top of kati's $(shell) usage list:
verbose: *kati*:   0.217 / 109 cat external/mesa3d/VERSION

Test: builds
Change-Id: Ic6f2903a093a3704e4749c9d8c66f7a739e74352
2 files changed