use `__APPLE__` instead of `__MACH__`

In [o3dgcTimer.h#L31](https://github.com/KhronosGroup/COLLADA2GLTF/blob/f7e8645/dependencies/o3dgc/src/o3dgc_common_lib/inc/o3dgcTimer.h#L31) there are some includes protected by an `#ifdef __MACH__`:

~~~C
#elif __MACH__
#include
#include
#else
#include
#include
#endif
~~~

It seems that this is intended for OSX/macOS.
However there are other implementations of the *Mach Microkernel*, notably `GNU Hurd`, which lacks the `mach/clock.h` include.
Therefore this file cannot be built on the Hurd.

If the define was indeed meant for OSX/macOS, I suggest to use `__APPLE__` instead:
~~~C
#elif __APPLE__
#include
#include
#else
#include
#include
#endif
~~~

Start with dependencies/o3dgc/src/o3dgc_common_lib/inc/o3dgcTimer.h around line 31 and inspect the platform-specific include guard. Verify the current condition fails on GNU Hurd, then confirm the macOS and fallback paths still select the expected headers. Done means the file builds on Hurd without changing the macOS behavior.

Written by the indexing model from the issue text.