...to avoid issue on Arch Linux

We need to grab the OS name before determining what the path should be.

According to Man Pages, timespec with std=c99 needs "_POSIX_C_SOURCE 199309L". Due a some kind of bug, "#define _POSIX_C_SOURCE 199309L" doesn't work, so we must use "-D_POSIX_C_SOURCE=199309L" in compiler options.

Also reviewed issue with stdbool when compiling with clang

Function names, code formatting...

BMFont loading has been improved to support unordered chars and extended
characters (up to 255)

I've renamed all makefile files to "Makefile" because they appear in the first files.

I've added "-ldl" library, it's necessary to compile the project.

I've cleaned the GNU/Linux compilation, now you can compile without useless parameters.

If you have raylib on standard directories ("/usr/local/include/raylib/raylib.h" for the header and "/usr/local/lib/libraylib.a" for raylib library) you can compile without edit the makefile.

I've changed all default target names with 'all'. Here there is the explanation: <https://www.gnu.org/prep/standards/standards.html#Standard-Targets>.