autoconf: Add PLUTO_FOUND flag

This fixes llvm.org/PR12250.

Contributed-by: Sam Novak <snovak@uwsp.edu>
Dependences: Further reduce the allowed compile time

llvm.org/PR19081 reports that the polly dependence analysis causes some h264
compilation to hang. We adjust the compute out, to ensure we do not block on
expensive dependence calculations.
Fix source code formatting