build: set OS=="android" for android builds

Work around the build name issues by instead manually adding the build
target for android builds using '-DOS=android'

This additionally resolves problems caused by the OS variable being
rewritten from "android" to "linux" in gyp. This causes errors, as
real-time support (-lrt) and pthread support (-lpthread) is not
available in the android NDK toolchain as libraries.

The functions present in these libraries are included automatically
during the build instead during the android compilation process.