* Fixed the build for web using CMake.

I found that the build for me was failing and I added some if defined checks in the core.c file where the glfwSetWindowAttrib was used. (error: implicit declaration of function 'glfwSetWindowAttrib' is invalid in C99 [-Werror,-Wimplicit-function-declaration])

I also changed some values in the toolchain file so that it correctly uses the .bat files when on windows.

* Cleaned up the additional variables (they are not important)

* Added more improvements to cmakelists

Added the option to use the system provided Emscripten toolchain to be more uniform with other libraries.

Fixed and issue which prevented example being built from cmake and also building with html extensions properly.

* Fixed ENUM to STRING because of a missed warning

We didn't use the latest version, but a compatible version, which is established by raylib-config-version.cmake.

It's ok to have a minimal version, so revert b7c0d5b6dd.

* Always use latest raylib version

* Always use latest raylib version

* Always use latest raylib version

At least on Linux, CMake seems to add automatically
the '-rdynamic' flag. As a result, Emscripten
generates a fat HTML file that does not work.
Removing it allows HTML+JS+WASM generation.

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.

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.

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. :)

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.

Closes #588.

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

src/core.c does explicitly skip including <camera.h> on Android
and tests fail. Therefore skip these failing tests.

Closes #507.

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.

Some people might find this handly