Fix KWSYS_PLATFORM_INFO_TEST when user removes tree
In commit 'Create KWSYS_PLATFORM_INFO_TEST macro' (2009-11-20) we implemented the macro to use a cache entry to avoid re-running the try_compile(). However, the output copied from the try_compile is needed on every configure. If the user wipes out the build tree but not the cache file then the try_compile() will not re-run to recreate the needed file. We address the problem by teaching the macro to run the try_compile() whenever its output file does not exist.