Stop referring to a removed make variable

Nothing uses the make variable GCCVERSION any longer, since we just
query the NDK for the default compiler.
Check for $(CXX).exe as well when checking for the android toolchain

This fixes building for android on windows.

Building for android on windows still requires MSYS for running make,
but running the ndk-build of the sample projects from within MSYS
doesn't seem to work (the OS variable from the OpenH264 makefile
interferes with the host system detection in ndk-build), so one still
needs to build it in two steps, first make OS=android ... libopenh264.so,
then building the sample projects manually.
refine arm code for sum of frame
Merge pull request #1273 from dongzha/RefineArmCodeForSumBlock

refine arm code for sum of frame
Merge pull request #1266 from mstorsjo/fix-readme

Stop referring to a removed make variable
Merge pull request #1270 from mstorsjo/android-tool-detect

Check for $(CXX).exe as well when checking for the android toolchain
Merge remote-tracking branch 'upstream/master' into travis.osx