Fully backwards compatible thanks to the new BASISU_LIB_VERSION/BASISD_LIB_VERSION.

The OpenCL dependency is kind of brutal since it hard-links to the shared library. Is there a nicer way to configure this other than CMAKE_DISABLE_FIND_PACKAGE_OpenCL?

To have at least one platform to test the library without OpenCL
support built in

Otherwise, if the BasisImporter is missing, this tests nothing

So that it's obvious that this works even without compiling from a
subdirectory

…folder

So there's some helpful debug output

This still downloads and extracts Basis even if not built on MSVC 2015
but it did that before as well, and making the preparation plus
unzipping optional would be really annoying.

To also apply the fixes required for GCC < 5

When it doesn't find a GPU device, it falls back to a CPU device and
prints a nice error along the way. Unfortunately, all OpenCL-related
errors are accompanied by an assert that can't be turned off. There's
BASISU_OPENCL_ASSERT_ON_ANY_ERRORS but that's always defined to 1 at the
top of the file...

So disable asserts via NDEBUG to actually get to use (and test) the
OpenCL code in debug builds.

…rectly

Apple's OpenCL implementation doesn't seem to support being used through
the ICD, so you have to compile against it directly or use a wrapper.

See also:
https://portablecl.org/docs/html/using.html#using-pocl-on-macosx
https://github.com/jrprice/ocl_icd_wrapper

Either Basis, or the OpenCL ICD loader, or the PoCL ICD, or the included
LLVM cause tons of sanitizer errors. Not much we can do here.