Restore possibly regressed CMP0018 logic
Refactoring in commit f4ff60a8 (cmMakefile: Make GetSafeDefinition return std::string const&, 2018-09-05) changed the treatment of the empty string in CMP0018 diagnostic logic. Restore the behavior.
Refactoring in commit f4ff60a8 (cmMakefile: Make GetSafeDefinition return std::string const&, 2018-09-05) changed the treatment of the empty string in CMP0018 diagnostic logic. Restore the behavior.
mentioned in commit 76a19eb6
·mentioned in commit 76a19eb6