Prepare for removal of platform builds.

The variable used to control the build of host GPU emulation
libraries is being changed from BUILD_EMULATOR_OPENGL to
BUILD_EMULATOR_HOST_OPENGL, so prepare for this by defining
both at the same time for now.

For more context, see http://android-review.googlesource.com/#/c/93980/

Change-Id: Ifa953646867291c1a3067dea02a95c11c3aa0a7f
1 file changed