Makefile compatibility with macOS

[lhaeger]

macOS ships BSD uname, which has no option "-o", so make fails. Change this to use bash ${OSTYPE} instead, which is platform independent.

[jpvlsmv]

In fact, I ran into a similar problem when not using the msys-compiled Windows systems... they don't have a uname command at all.

How about python -e 'import sys;print(sys.platform)' ?

[arsenetar]

@lhaeger, I think I like the suggestion from @jpvlsmv as it only depends on python to be installed (which we need anyhow), there may be systems without bash. I think that would be more platform independent in the long run unless I am missing something.

[lhaeger]

Whatever works is fine with me. I‘ll close this PR and leave that up to you.

[arsenetar]

Went ahead with ${PYTHON} -c "import platform; print(platform.system())" as it seemed to give a nice "Windows", "Linux" output where sys.platform gave win32 win64 etc. It seemed like the platform.system() gave easier to handle output.