Fixes#1370
Adds a simple helper assertion to tell the user how to enable BT if it's
not enabled, instead of some odd compilation warnings about undefined
functions.
Fixes#1356
Mac and Windows have case-insensitive filesystems, so the will find
the internal (all-lowercase) "btstack.h" and not the library's "BTstack.h",
causing compilation errors.
Rename the library and header to avoid the issue.
Earle F. Philhower, III