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
diff --git a/Makefile.android b/Makefile.android
index 6d1ca2d..586797d 100644
--- a/Makefile.android
+++ b/Makefile.android
@@ -339,7 +339,9 @@
     $(error Cannot find GPU emulation sources directory: $(EMULATOR_EMUGL_SOURCES_DIR))
   endif
 
+  # TODO(digit): Remove the line below.
   BUILD_EMULATOR_OPENGL := true
+  BUILD_EMULATOR_HOST_OPENGL := true
   include $(EMULATOR_EMUGL_SOURCES_DIR)/Android.mk
 endif