The check in the configure script (which breaks the build with clang now -
apparently it was not tested before being committed)
The script provides instruction on how to ignore the check for compilers which
don't support the 'standard' gcc behaviors. That worked on my system when I
tested it. I put that option in for old versions of gcc and because the latest
info I managed to find for clang was that it didn't support characterset
specifier flags and didn't check what characterset it was writing using for
string literals.