Support Bullet with double precision instead of one with single precision

Bullet with single precision causes some bugs in OpenMW due to lack of collision, so it is better to do not support it at all.

Also it would be nice to make use of try_compile to check if the system version of bullet is compiled with double-precision, instead of having the linking fail after the compilation with a confusing error message. Kudos to @AnyOldName3 for the idea.