Exclude threadasm.S from build if CMake version is less than 2.8.5.

Assembler support was rewritten in CMake 2.8.5. The new functionality
is required to assemble the file, otherwise a bunch of error messages
is generated. The solution is to exclude the file from the build.
This is only a problem for non-x86 platforms (PPC, Mips).

General advise is to use at least CMake 2.8.5. This fixes #464.
Only define version Android if compiling for this environment.

Android is a variant of Linux. In order to simplify the version coding,
only version Android is defined (version linux and version Posix are not).