Add mention of problems with ICD on Mac OS X

Fix pocl#984 - broken link in README.Windows

also update the document with status of Windows build

Update documentation for upcoming 1.8 release

Revert reported platform to 2.0

This is necessary as we don't yet implement all required
runtime APIs for 3.0 (including 2.1 and 2.2)

Update documentation for release 1.7

Fix memleak in tests/runtime/test_event_double_wait.c

Temporarily disable LLVM 12 support (for 1.6 release)

1.5 docs updates

1.5 docs updates

Fix some compiler warnings