Right now, every re-run of configure re-evaluates whether a
static analysis tool is in use.  But if you run configure under
coverity, make a tweak, and then do an incremental rebuild with
gcc but not coverity to test the tweak, then rerun a build under
coverity, then configure does not get rerun, and static analysis
ends up with lots of false positives.

This patch caches the static analysis result, and also makes it
easier to force static analysis even if the existing checks are
insufficient to detect newer versions of the static analyzer tools.

* configure.ac (lv_cv_static_analysis): New cache variable.