I use a command:
$ g++ -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/usr/include/c++/4.2.1
Apple LLVM version 8.0.0 (clang-800.0.42.1)
Thread model: posix
The short answer is you are using Apple's LLVM pretending to be gcc/g++. This is not a huge problem. One place where I was annoyed enough was the fact that gcc's
-march=native optimizations just do not work with LLVM which resulted in about 50% worse performance in a rather CPU-intensive program I was running.
If you get the real thing working, you might also want to build dedicated GNU binutils etc.