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.