Since commit 9199f7c6 (Disable arch-specific try_run in CMake itself,
2009-12-14, v2.8.2~567) we've abused an undocumented debugging feature
to override the builtin `try_run` command in CMake's own build with a
wrapper macro.  However, we've also long discouraged use of this feature
by other projects.  The purpose of the original change is outdated and
of limited use anyway, so just drop it.