This commit switches several internal (implied) Makefile variables to
lower cases.  This follows the variable naming convention recommended by
GNU Makefile Manual.

Change-Id: I79819ef0cfd3828107dab665118f1511ef8a8337
Reviewed-on: https://swiftshader-review.googlesource.com/20848Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Logan Chien <loganchien@google.com>