Support external GLFW usage
Renamed some flags for consistency

I think is clearer this way...

Trace log messages could be configured with this function to select wich
ones are shown

That function depends on SavePNG() and forces platform check on textures
module, that behaviour should be changed, platform definition should be
only required on core module...

OpenAL audio backend is being forced on HTML5 and OSX

Audio module requires a complete formatting review....

Monotonic time since InitWindow() could be retrieved with this function.

Reviewing GetTime() functionality

Some programs like tools could not require audio support

If shader file could not be found on loading it crashed, instead added
fallback to default shader!

Let the user choose if using rres external library

Less maintenance overhead that way,
given that make/CMake now also do library versioning.

See #401 for the discussion.
Also bumps version number to 1.9.2 without the -dev,
because neither ELF nor MachO like such a suffix.
The -dev suffix will have to be restricted to the git tags.

Updated to latest GLFW library and working on Wayland support, still
looking how to implement it on raylib because it just exposes
PLATFORM_DESKTOP and defaults to X11 windowing system on Linux...

OpenAL Soft backend is still available in audio module, I'm thinking if
exposing it for building in some way or just left it there for advance
users to switch to it manually in case of necessity...

Added dr_wav for a future use

Updated raylib version to 1.9-dev for development pourposes.

Next raylib version is planned to implement a big amount of changes, so
the version bump.