Allow platform-dependence in targets?

[GPMueller]

This would allow a lot of abstraction layers, such as GLFW, where the platform-dependence cannot be avoided, to be built quite naturally with clang-build. The goal should again be that the easiest solution is to have no platform-dependence, but to allow it if actually necessary.
Aside from abstraction layers, one sometimes has to apply platform-specific linkage, e.g. to the pthreads library on unix systems.

The following would allow it to be composable and non-intrusive:

[mytarget]
target_type = "shared library"

sources = ["src/common.cpp"]
[mytarget.flags]
compile = ["-DSOMETHING"]

[mytarget.osx]
sources = ["src/osx/platform_specific.cpp"]
[mytarget.osx.flags]
link = ['-lpthread']

[mytarget.linux]
sources = ["src/linux/platform_specific.cpp"]
[mytarget.linux.flags]
link = ['-lpthread']

[mytarget.windows]
sources = ["src/windows/platform_specific.cpp"]

The corresponding osx, linux and windows subdicts would be merged into the main one if the corresponding platform is detected. This of course only makes sense for lists, i.e. include_directories, sources and flags (what about dependencies?).

[GPMueller]

Added platform-dependent flags with 3e75731 on branch feature-improved-config. The approach there should be rewritten to take mytarget.osx.flags instead of mytarget.flags.osx, what I wrote above seems more general and therefore better.

[GPMueller]

Implemented correctly with 2d54e0a on feature-improved-config.