Since commit v2.8.8~176^2 (try_compile: Use random executable file name,
2012-02-13) the length of the test executable name in generated
try_compile projects has been longer and unpredictable.  With Visual
Studio on windows, the tools try to create paths like:

 CMakeFiles/CMakeTmp/$tgt.dir/Debug/$tgt.tlog/$tgt.lastbuildstate

With the target name repeated up to 3 times, we must make it short and
of consistent length to avoid overrunning the 260 character limit
imposed by VS tools.