Use cmake's target_sources(), and general cmake file cleanup.

[floooh]

Second attempt to use the 'new-ish' cmake target_sources() command. Main difference is that an (empty) target is now already created in the fips_begin_* calls, not in fips_end(). This allows a much simpler internal structure because target items can now be added immediately in the other fips functions instead of being stored until fips_end_*() is called.

Cleanup changes:

• merge the platform-specific cmake helper file into a single fips_platform.cmake file
• fips_project() is deprecated, use project() instead
• fips_finish() is deprecated, can be removed entirely
• fips_begin_module() and fips_end_module() are deprecated, and are now aliases for fips_begin_lib/fips_end_lib
• likewise, fips_deps() is now an alias for fips_libs() (but not deprecated)
• fips_include_dirs() is deprecated, use target_include_directories() instead
• fips_add_subdirectory() is deprecated, use add_subdirectory() instead
• fips_vs_warning_level() and fips_vs_disable_warning() are deprecated, use target_compile_options() instead (and the default warning level for MSVC configs has been bumped to /W3)

(all deprecated functions now print a warning message, but still work - but will be removed in the next cleanup round)

TODO:

• fips.yml: header dirs and module exports in fips.yml should be deprecated, this should all be defined via cmake => add deprecation messages for this! (header exports are needed for header-only libs, but figure out if module exports are actually still needed) => module exports make sense because they basically just trigger the generation of an 'add_subdirectory()' in the toplevel project, the naming is just a bit unfortunate (should be exported targets instead)
• FIPS_OSX_STANDARD_FRAMEWORKS should be removed eh, just keep it in, Win32 has tons of standard link libraries too
• add a fips_target() function which returns the current target between begin/end, and otherwise fails (but do NOT clear the CurTargetName variable in fips_end() for backward compatibility, instead use a separate flag) no such thing as return values in cmake, and the alternatives would be too messy
• add a FIPS_APPLE_CLANG variable to differentiate between vanilla and Apple clang
• update documentation