libwebsockets

gcc: support -fanalyzer static analysis

Starting with gcc 10 (in fedora 32) there's a new static
analyzer built into gcc you can enable with -fanalyzer.  It
doesn't slow compilation much, but it's a bit hit and miss,
it only analyzes each compilation unit standalone so it
reports issues that can never happen.

Enable it if the option LWS_WITH_FANALYZER is enabled and
cmake can see the actual compiler supports it.