Update Makefile in `examples/`

We need to grab the OS name before determining what the path should be.

Now it should be coherent with OpenGL math standards

Exposed some raymath useful functions to raylib API

Consistent with similar functions in raymath

Still shader dependant, locations set should be moved out

SetMaterialTexture() and UnsetMaterialTexture()... too shader
dependant...

Corrected compilation for OpenGL 1.1
Review mesh data uploading to GPU

Moved PBR material loading to example, right decision?

Able to make a .dylib for OS X

That's just a reference file, needs review

Problems when compiling with VS2015
Probably DEBUG(...) macro is not the best naming...

...to avoid possible conflicting symbols

Fix for OBJ loading

Fix some typos (lenght -> length)

Removed an unwanted file and fixed an example

Add a density parameter to GenImageRadialGradient

Add more image generation functions: radial gradient and perlin noise

Add the SaveImageAs function

Add some image generation functions

Namely:
- GenImageHorizontalV
- GenImageHorizontalH
- GenImageChecked
- GenImageWhiteNoise
- GenImageCellular

The gradient implementation may be a bit naive, for example it doesn't do any gamma correction.