Fix for musl systems and expand context cracking capabilities

[Jack-Punter]

The 4coder build fails on non glibc linux systems due to non-standard GNU extensions used in the linux platform layer. This PR is a continuation of #56 which was closed due to the submitter deleting his repository.
It:

• removes the use of GCC extensions
  • replace strndupa with a strcpy into a fixed sized buffer (with size PATH_MAX)
  • replace localtime with mktime
• Addresses a TODO in the linux platform's system_get_cannonical function
  • replaces custom string with realpath
• update context cracking in 4coder_base_types.h to be more thorough, should also enable the use clang on windows and linux rather than just msvc and gcc respectively
• Add || defined(__linux__) to linux os cracking as musl defines that over __gnu_linux__