In r483285, compiler:c++11-lang was added to USES in order to unbreak the
port on GCC-based architectures, while actually the code predates 2011 by
couple of years. Turns out that the build against GCC was failing because
of the -Wno-c++11-narrowing option added back in r474518 to unbreak the
build against Clang 6. Fix the underlying problem with the code instead,
so we can remove offending option from CXXFLAGS and stop bogusly require
C++11-capable compiler just because of that option.

Tested on: Mac mini G4 (powerpc, gcc-4.2.1-based)