I tried to compile my c++ code with c++11 features on GCC 4.2.1 which is default on my MacBookPro3,1 with MacOS 10.8.5. I used random library, so I needed newer version of GCC, 4.7 at least to be more precise. I already have MacPorts on this machine (installation guide can be found here). So:

/opt/local/bin/port install gcc5

No I can compile:

/opt/local/bin/g++-mp-5 -std=c++11 main.cpp

Even that you use GCC 5.5.0 you need to pass flag to enable c++11.

If someone thinks that this was too easy, then lets try to do this on MacOS 10.4.11 on PowerBook3,4. First thing is to install XCode 2.5 which suppose to be the last one for 10.4 operating system. You can grab it from xcodereleases.com after signing in with your Apple ID. Install XCode and then download MacPorts 2.8.0. Next, install GCC 5 using same command as previously. This time it will take forever to build it as it has way less powerful hardware than newer MacBookPro3,1. During that time it got so hot so I could not even touch keyboard. Iit was around 50 degrees Celcius while being cooled and over 65 degrees without cooling.

sudo /opt/local/bin/port -f deactivate libunwind-headers
sudo /opt/local/bin/port install apple-gcc42
sudo /opt/local/bin/port install gcc5

Unfortunately GCC 5 is not supported and you need to install GCC 4.8 or at least 4.7. Prepare one day for building libgcc6, another day for libgcc7, one more for GCC itself and half a day for other stuff.