Continue Linux port

Created by: Zegeri

This a big pull request. Fortunately, most of the changes are the inclusion of a platform independent delimiter for file paths and the renaming of header includes for the case-sensitive Linux. An incomplete list of general changes:

• Compilation support of most of the project in GCC and Clang
• More prevalent use of SDL ( ac669b5f )
• Better error messages ( 6cfd99fb )
• Running the game up to GLSL Shaders compilation if you use Mesa drivers
• Running the game up to xrScriptEngine initialization if you use NVIDIA propietary driver
• Some code simplification ( cace3a37 )

Potential issues:

• The usage of sscanf instead of sscanf_s might cause trouble.
• The Linux build might take too much time and exhaust Travis limit.