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.