M4/clang.m4 : Minor improvements.
src/wav.c : Fix parsing of malformed LIST chunk.

Example file provided by Joshua Rogers on the libsndfile-devel
list had the following LIST chunks:

   LIST : 140
     INFO
       ISFT : gvfs-cdda using libcdio 0.81 i486-pc-linux-gnu
       *** ISTR : 12
       IART : Deep Purple
       INAM : Fireball
       *** IPRT : 2
       *** IFRM : 4
   LIST : 64
       *** MID  : 1414676810

Obviously, a LIST chunk 64 byte in size cannot contain another chunk
that is very much larger.

The fix was to make sure that inside a LIST chunks, any unknown chunk
types with impossibly large sizes get handled correctly.
configure.ac : Allow -Werror to be switched on for Clang.

Commit d7cc3dd0a4 allowed the detection of Clang masquerading as GNU GCC
which was required because Clang is not the same as GCC. However, that
change prevented configuration with --enable-gcc-werror.

This fix changes the configure option to --enable-werror and will allow
it to be enabled for any compiler that accepts the -Werror flag.
configure.ac : Report if C compile is Clang.