CTest: Teach ctest_coverage() to read CTEST_COVERAGE_EXTRA_FLAGS

Use it to set the CoverageExtraFlags value just as COVERAGE_EXTRA_FLAGS
does in the CTest module for creating DartConfiguration.tcl.

Source/CTest/cmCTestCoverageCommand.cxx

@@ -25,7 +25,8 @@ cmCTestGenericHandler* cmCTestCoverageCommand::InitializeHandler()
 {
   this->CTest->SetCTestConfigurationFromCMakeVariable(this->Makefile,
     "CoverageCommand", "CTEST_COVERAGE_COMMAND");
-
+  this->CTest->SetCTestConfigurationFromCMakeVariable(this->Makefile,
+    "CoverageExtraFlags", "CTEST_COVERAGE_EXTRA_FLAGS");
   cmCTestCoverageHandler* handler = static_cast<cmCTestCoverageHandler*>(
     this->CTest->GetInitializedHandler("coverage"));
   if ( !handler )