Refactoring in commit f4ff60a803 (cmMakefile: Make GetSafeDefinition
return std::string const&, 2018-09-05) accidentally changed the logic
for target artifact prefix and suffix names such that setting a PREFIX
or SUFFIX target property would cause an empty value to be used. Revert
that part of the change and use a simpler alternative. Add a test case.
Reported-by: Alan W. Irwin <irwin@beluga.phys.uvic.ca>
