Physac wasn't working on OS X.  It looks like the necessary timing code wasn't being compiled in by the preprocessor.

fixes #340

To multiply Vector3 by Vector3

For consistency!

Now it works great with reviewed maths

Some other tweaks...

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?

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

...to avoid possible conflicting symbols

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

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

If we compile with c99 without gnu extensions (gnu99) we need this
define, to have CLOCK_MONOTONIC and similar macros available

This should always be the number of the raylib release.
Especially when the new version is ABI incompatible the major version
should be bumped.

Using the library() function instead of shared_library() allows changing
of the library type via the default_library option.
This allows for easy change between static and dynamic library building.

Use 'meson --default-library=static builddir' to build as static, if no builddir yet exists.
Use 'mesonconf -Ddefault_library=static builddir' to change the type for
an existing builddir.