The build system for the x86_64, x86 and armv7 architectures has been tested on API 21 and for the x86_64 and arm64 architectures on API 24.  Of these combinations of architecture/api, only x86_64 on API 21 builds correctly today, the others were building correctly few weeks ago but recent changes in Python seem to have broken those builds.

To fix this temporarily:
* Build the 32 bits architectures (x86 and armv7) with the attached issue29619.patch. This is a hack, the problem will be fixed in issue 29619.
* Build on API 24 with the attached android-api-24.patch. This is another hack, I will enter a new issue for those problems which are of a kind that is recurrent with the NDK: a function implemented in libc and whose declaration is missing in the NDK headers, a problem that is supposed to be fixed by UnifiedHeaders.