Update the claim "GCC does not require a C++ compiler to build it" on comparison page

On the "Clang vs Other Open Source Compilers" comparison page
(http://clang.llvm.org/comparison.html), one of the pros of GCC is
"GCC does not require a C++ compiler to build it."

This is no longer true: "GCC now uses C++ as its implementation
language. This means that to build GCC from sources, you will need a
C++ compiler that understands C++ 2003"
(GCC 4.8 Release Series — Changes, New Features, and Fixes - GNU Project).

Could you update the comparison page? Thanks.

Done in r176515, thanks.