psbuild should support a way to force certain libs to be built from extlibs

[welling]

There is a version of autoconf in the default path on Odyssey, but it's out of date. The test for the presence of autoconf within pschecklibs finds it, however, so the up-to-date version included in the IPP distro does not get built. If psbuild supported an argument like '-force autoconf' that would build the included version I'd be able to easily work around this problem.

[Paul Price]

Use "pschecklibs -force libname"