Generate and install pkg-config pc file and allow static/shared builds in one go

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"

Add AppVeyor CI for automatic Windows Builds

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.

Fix macOS build of new rglfw.c approach

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

Added  clean to Makefile for FreeBSD

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

Store the default shaders

libEGL and libGLESv2 have been renamed on latest Raspbian Stretch
version (also included on latest Jessie upgrade).

It seems to be done to avoid conflicts between official Mesa libs and
Broadcomm provided ones.

USE_DEPTH_RENDERBUFFER on OpenGL 2.1

Not tested, could have errors

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

Have SetShaderValue(i) take a const pointer

Could correct possible issues on OpenGL 1.1

C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin>dumpbin.exe
D:\GitHub\raylib\release\libs\win32\msvc\raylib.dll /DEPENDENTS

Microsoft (R) COFF/PE Dumper Version 14.00.24215.1
Copyright (C) Microsoft Corporation.  All rights reserved.

Dump of file D:\GitHub\raylib\release\libs\win32\msvc\raylib.dll

File Type: DLL

Image has the following dependencies:

KERNEL32.dll
glfw3.dll
OpenAL32.dll

Summary

4000 .data
1000 .gfids
23000 .rdata
5000 .reloc
1000 .rsrc
68000 .text
1000 _RDATA

Added support for raylib DLL building on MSVC

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

Corrected issue with RPI_CROSS_COMPILE