Rewire cmake internals for cmake's target_source()

[floooh]

...instead of delaying the cmake target creating the the fips_end_*() function, an empty target is now created in the fips_begin_*() macros, and adding sources, options, dependencies etc is now directly to the respective cmake target_*() functions instead of first gathering everything in variables. This simplifies the cmake scripts quite a bit and allows to directly use standard cmake commands (like target_sources() or target_compile_options() inside the fips begin/end block.

[floooh]

Yeah it's a weird feature indeed.