Use += for all submodules INCLUDE variables

This makes it easier for platform specific makefiles to add
things to some of them. (build/platform-android.mk already does this
for a few submodules, e.g. GTEST_INCLUDES and CODEC_UNITTEST_INCLUDES
already use +=.)
Merge pull request #1063 from mstorsjo/unify-makefile

Use += for all submodules INCLUDE variables