Otherwise compiling rlgl is too spammy. While at it remove
the now unneeded CheckIncludeFiles.

The example requires pthreads and now C11 <stdatomic.h>, thus skip it
if either isn't available.

This also fixes the current Travis CI build failure for the
Windows configuration.

A plain variable is insuffecient for inter-thread communication. Both the
compiler and the processor may reorder accesses. The compiler could even
cache dataLoaded with the result that STATE_FINISHED becomes unreachable.

Fix this by using C11 atomic_bool, which guarantees sequential consistency.

This fixes #827.

And while at it, use a switch clause to make the state machine
structure clearer.

Some examples included in this batch require the included libraries: `easings.h` and `raygui.h`. Examples included:

 - shapes_bouncing_ball
 - shapes_collision_area
 - shapes_following_eyes
 - shapes_draw_circle_sector (requires raygui.h)
 - shapes_draw_rectangle_rounded (requires raygui.h)
 - shapes_draw_ring (requires raygui.h)
 - shapes_easings_ball_anim (requires easings.h)
 - shapes_easings_box_anim (requires easings.h)
 - shapes_easings_rectangle_array (requires easings.h)

Data loading in a second thread with progress bar in main thread

Added walls collision check

Found some issues when building for web using latest emscripten 1.38.30, traced the error and found that eglGetProcAdress does not return function pointers for VAO functionality, supported by extension.

It requires more investigation but now it works (avoiding VAO usage)

Added experimental network module

Already integrated into raylib

- No default VR device parameteres inside raylib
- VR device parameter should be provided by user
- VR distortion shader should be provided by user

Some tweaks

Issue was related to vertex tangent attibutes not uploaded to GPU, a quick solution was implemented for new vertex attributes loading for already existing meshes... I don't like it specially but it will work for now.

Multiple functions added and some reviewed to adapt to the new multi-mesh, multi-material and animated models.

They could be useful for some people...

No examples built for the Web platform functioned properly due to lack
of resources in the virtual file system provided by emscripten.

This patch addresses this problem by adding
'--preload-file local_path/resources@resources' emcc option to link
flags whenever necessary.

The default memory limit for emscripten applications is 16 mergabytes,
which might be to little for some examples, especially given that the
resources are also included in that limit.

Normally, using this option disables some asm.js optimizations, but
there's no such cost for WebAssembly.

Planning to promote raudio module as a simple and easy-to-use front-end for the amazing mini_al library, so the name change.

Name comes from raylib-audio but in spanish it also remembers to word "raudo", meaning  "very fast", an analogy that fits perfectly to the usefulness and performance of the library!

Consequently, raylib version has been bumped to 2.4-dev.