option to speedup running of configure script

To anyone tired of waiting for the configure script to run: try the -C option.

This creates a configure cache (config.cache) which stores the result of each test. When a test is to be run, configure first checks the cache, and if the answer is there, the test is not run. This helps a lot when rerunning configure, but also running it just one time. This is because configure reruns a lot of tests multiple times.

This also speeds up the libtool and udunits configure runs, which will use the answers from the netcdf configure run if they are available.