ThirdParty: Darwin platforms: using Mac Ports compilers only when specifying System compiler option

etc/bashrc

@@ -247,6 +247,8 @@ Darwin)
     # Make sure that binaries use the best features of the used OS-Version
     export MACOSX_DEPLOYMENT_TARGET=`sw_vers -productVersion`
 
+    if [ "$compilerInstall" == "System" ]
+    then
         # Use Mac-Ports-Compiler instead of Apple-gcc-4.2
 	case $WM_COMPILER in
 	Gcc43)
@@ -265,6 +267,7 @@ Darwin)
             export WM_COMPILER=
             ;;
         esac
+    fi
 
     ;;