They were disabled because they failed to build,
but this patch set fixes the build on Linux and macOS.

This doesn't apply to the AppVeyor build on Windows yet;
it currently fails at linking with OpenAL.

cmake --build . --target package # or make package if make is used

can now be used to create binary packages for raylib.

AppVeyor and Travis CI are configured to push the artifacts
that result from building git tags to the related Github releases page.

After installation, compiling new programs is possible with
    $ cc game.c `pkg-config --static --libs --cflags raylib`
or
    $ cc game.c `pkg-config --libs --cflags raylib`
depending on configuration

Also adds following configuration options:
- WITH_PIC "Compile static library as position-independent code"
- STATIC_RAYLIB "Build raylib as a static library"
- MACOS_FATLIB  "Build fat library for both i386 and x86_64 on macOS"

Suppresses 88 of the 213 warnings reported when compiling with
MSVC 2015 on AppVeyor.

We already have automatic Linux and macOS build via Travis CI.
This adds the same for Windows x86 and x86_64 with both
Microsoft Visual Studio 2015 as well as MinGW-w64.

There have been two problems:

* GLFW itself was compiled with the definitions for compiling
_against_ GLFW (fixed by removing requirement for external glfw)

* rglfw.c was being compiled as C code, although it includes
Objective C files.

This _might_ break the Windows build, needs to be checked.

Fixes #391, but as noted I'd prefer though a separate source directory
and build script for GLFW.

Feature macros need to be defined before #including any headers,
preferably through the build system, but this is good enough.

Fixes a compile error on my fork's Travis CI.

By default, we look for texture1 for LOC_MAP_SPECULAR and texture2 for
LOC_MAP_NORMAL

Full sources already included and compiled with raylib (rglfw)

Only required by examples

Just in case of legal conflict with public domain software, added MIT
alternative license.

...to define proper values

Compiling GLFW library with raylib avoids external dependencies, this
way we solve version problems in some platforms

Now header is truly multiplatform...

Actually still a small pending check on XBOX gamepad controls that
hopefully will be removed with next GLFW 3.3

USE_DEPTH_RENDERBUFFER on OpenGL 2.1

On RPI, after latest drivers update, models were not rendered properly
cause of that...

- Review RPI compilation (core_drop_files not supported)
- Review ImageFormat(), some issues
- GetTextureData() reviewed for RPI

Corrected issue with RPI_CROSS_COMPILE

Improve second option to retrieve texture data from fbo...

Still requires some testing...

...while reviewing GetTextureData() functionality on OpenGL ES 2.0

- Added new function: GenImageColor()

ImageDraw() should be reviewed... specially alpha blending...

To be more generic and configurable

Variable t was read and modified without interleaving sequence points,
technically undefined behavior. Report by Clang's -Wunsequenced