nits: replace some "c" instances with 'c'

Source/CTest/cmCTestBuildHandler.cxx

@@ -541,11 +541,11 @@ void cmCTestBuildHandler::GenerateXMLLaunched(cmXMLWriter& xml)
     const char* fname = launchDir.GetFile(i);
     if (this->IsLaunchedErrorFile(fname) && numErrorsAllowed) {
       numErrorsAllowed--;
-      fragments.insert(this->CTestLaunchDir + "/" + fname);
+      fragments.insert(this->CTestLaunchDir + '/' + fname);
       ++this->TotalErrors;
     } else if (this->IsLaunchedWarningFile(fname) && numWarningsAllowed) {
       numWarningsAllowed--;
-      fragments.insert(this->CTestLaunchDir + "/" + fname);
+      fragments.insert(this->CTestLaunchDir + '/' + fname);
       ++this->TotalWarnings;
     }
   }

Source/CTest/cmCTestCVS.cxx

@@ -157,7 +157,7 @@ private:
       if (!this->Rev.Log.empty()) {
         // Continue the existing log.
         this->Rev.Log += this->Line;
-        this->Rev.Log += "\n";
+        this->Rev.Log += '\n';
       } else if (this->Rev.Rev.empty() &&
                  this->RegexRevision.find(this->Line)) {
         this->Rev.Rev = this->RegexRevision.match(1);
@@ -168,7 +168,7 @@ private:
       } else if (!this->RegexBranches.find(this->Line)) {
         // Start the log.
         this->Rev.Log += this->Line;
-        this->Rev.Log += "\n";
+        this->Rev.Log += '\n';
       }
     }
     return this->Section != SectionEnd;

Source/cmComputeLinkInformation.cxx

@@ -1003,10 +1003,10 @@ std::string cmComputeLinkInformation::NoCaseExpression(const char* str)
     if (*s == '.') {
       ret += *s;
     } else {
-      ret += "[";
+      ret += '[';
       ret += static_cast<char>(tolower(*s));
       ret += static_cast<char>(toupper(*s));
-      ret += "]";
+      ret += ']';
     }
     s++;
   }

Source/cmExtraKateGenerator.cxx

@@ -273,7 +273,7 @@ std::string cmExtraKateGenerator::GenerateProjectName(
   const std::string& name, const std::string& type,
   const std::string& path) const
 {
-  return name + (type.empty() ? "" : "-") + type + "@" + path;
+  return name + (type.empty() ? "" : "-") + type + '@' + path;
 }
 
 std::string cmExtraKateGenerator::GetPathBasename(