Correcting details that pop-up when testing the different platforms

Generates:
Win32:  raylib.dll, libraylibdll.a (import library)
Linux:  libraylib.so

This commit restores the original method to compile all modules, but fix
prerequisites.

The first target allow makefile to install the dev files (static library and
header) to standard directories on GNU/Linux platforms; the second allow it to
unistall (remove) the dev files.

It needs lot of improvements.

Set makefile to default OpenGL 3.3 compilation

Moved all external libraries used by raylib to external folder inside
raylib src. Makefile has already been update and also the different
includes in raylib modules.

I've added .PHONY targets and fixed "clean" recipe.

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

[rlgl] Extensions strings loading was redone to avoid a Segmentation
Fault on Raspberry Pi