Check if detected libpython.so can be dlopen-ed

Check if detected libpython.so can be dlopen-ed

A somewhat better solution for the libpython.so detection saga.
Unfortunately it has not presented itself and had to be painfully
extracted from a series of trial and error experiments. No living
beings were harmed in the process, though.

Many thanks to Paul for the help with testing this.
(cherry picked from commit df8a1d7e766dea1c48a47d6a7766e20922a78cab)