CMake: Major cleanup to support find_package(raylib)

With the recent CMake cleanup, getting raylib's dependencies for use
when building rlgl_standalone is quite straight forward, so lets enable
it again.

Fixes #508, just properly this time. :)

Homebrew doesn't ship 32-bit libraries anymore, so using both
-DUSE_EXTERNAL_GLFW and -DMACOS_FATLIB with a Homebrew GLFW would fail.
With -DUSE_EXTERNAL_GLFW=OFF, this is not a problem, but I think keeping
it off by default makes more sense. If someone wants universal
libraries, they can just toggle it.

PLATFORM_WEB is not the only platform that doesn't define
GLFW_TRANSPARENT_FRAMEBUFFER, it's also recent enough that it's not
included in the most recent GLFW 3.2.1 release, so this error can creep
up when using USE_EXTERNAL_GLFW.

Remove that link_libraries_to_executable() hack and defines a proper
raylib target that can be used with target_link_libraries.

The same target is also available for external (user) code by using
find_package(raylib).

This results in:

- Remove hardcoded build directories from examples and games CMakeLists.txt
- Allow rlgl_standalone and other special examples to be built easily
- Allow CMake projects to find_package(raylib instead of fiddling with pkg-config
- Makes code a little more maintainable
- Fixes #471, #606.
- Makes code less confusing by removing the double use of PLATFORM (#584).

Note that this is still not _The Right Way_(TM), because normally
raylib-config.cmake (or its includes) would be automatically generated.
I didn't manage to get that to work though, so I went the easier route
of just wrapping pkg_check_modules for consumption by find_package.

also makes one configuration shared-only

dr_flac is not built on i386, because GCC 4.8 complains about asm("cpuid"
clobbering ebx, as it's using ebx for PIC.

Instead of downloading a newer GCC (and multilib),
disable FLAC for that one configuration...

Fix bug in LoadMusicStream

free() and NULL at the end.

Deal with failed LoadMusicStream

It would be good to have a way to continue execution even when loading a stream didn't work.
 Free music and set it to NULL in case something went wrong, so that users can check for it.

Complete raw_audio_stream example

Update Builder example

Update Builder example for raylib 2.0.0.
External OpenAL and GLFW are not required anymore.

1. Always synthesize a complete frame of audio, using a second buffer (this prevents gaps in playback)
2. Sine is computed correctly, with an adjustable frequency
3. User can modulate frequency in real-time with mouse
4. Entire audio buffer data is shown, visually demonstrating how sine changes in wavelength

Fix rlgl.h to be used as a standalone

Closes #588.

Mimic the Makefile by outputting html + js instead of LLVM IR.

Still a work in progress but it already works...
Current riqm API could be simplified...

Fixes: 1d0ba96 ("Add GLFW-installed-but-using-our-GLFW-anyway configuration")

as well as -DUSE_EXTERNAL_GLFW.

Nips issues like #584 in the bud.

Remove period after exclamation

Fixes https://www.reddit.com/r/gamedev/comments/90z6fy/raylib_20_released/e2v5qpl/ :)

Convert window render sizes to floats

GLSL 1.10 is typesafe ([PDF specs](https://www.khronos.org/registry/OpenGL/specs/gl/GLSLangSpec.1.10.pdf), page 22), so this shader will not load properly during build.

It's not a super important change, but I came across it while playing with the examples on my pi.

Examples can be compiled for web with no code change at all! Usually
examples need to be refactored for web... using emscripten code
interpreter (emterpreter), it can manage synchronous while() loops
internally... as a downside, execution is very slow...

Not properly configured, use VS2015.UWP as reference