Fix regression in target output file naming logic

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>

1 files changed, 8 insertions, 9 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index f563dd8..434e0a3 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -3488,13 +3488,12 @@ void cmGeneratorTarget::GetFullNameInternal(
   }
 
   // if there is no prefix on the target use the cmake definition
-  std::string targetPrefix2, targetSuffix2;
   if (!targetPrefix && prefixVar) {
-    targetPrefix2 = this->Makefile->GetSafeDefinition(prefixVar);
+    targetPrefix = this->Makefile->GetSafeDefinition(prefixVar).c_str();
   }
   // if there is no suffix on the target use the cmake definition
   if (!targetSuffix && suffixVar) {
-    targetSuffix2 = this->Makefile->GetSafeDefinition(suffixVar);
+    targetSuffix = this->Makefile->GetSafeDefinition(suffixVar).c_str();
   }
 
   // frameworks have directory prefix but no suffix
@@ -3502,19 +3501,19 @@ void cmGeneratorTarget::GetFullNameInternal(
   if (this->IsFrameworkOnApple()) {
     fw_prefix = this->GetFrameworkDirectory(config, ContentLevel);
     fw_prefix += "/";
-    targetPrefix2 = fw_prefix;
-    targetSuffix2.clear();
+    targetPrefix = fw_prefix.c_str();
+    targetSuffix = nullptr;
   }
 
   if (this->IsCFBundleOnApple()) {
     fw_prefix = this->GetCFBundleDirectory(config, FullLevel);
     fw_prefix += "/";
-    targetPrefix2 = fw_prefix;
-    targetSuffix2.clear();
+    targetPrefix = fw_prefix.c_str();
+    targetSuffix = nullptr;
   }
 
   // Begin the final name with the prefix.
-  outPrefix = targetPrefix2;
+  outPrefix = targetPrefix ? targetPrefix : "";
 
   // Append the target name or property-specified name.
   outBase += this->GetOutputName(config, artifact);
@@ -3533,7 +3532,7 @@ void cmGeneratorTarget::GetFullNameInternal(
   }
 
   // Append the suffix.
-  outSuffix = targetSuffix2;
+  outSuffix = targetSuffix ? targetSuffix : "";
 }
 
 std::string cmGeneratorTarget::GetLinkerLanguage(