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.