Remove -Wc++-compat for the sake of the CRAM code

The CRAM code is not currently C++ compatible, mainly due to void* to
other* conversions (mostly from malloc()/realloc()) without explicit
casts, so -Wc++-compat causes lots of warning noise.

We probably use -Wc++-compat so that Windows users have a chance to
compile samtools via MSVC's compile-as-C++ workaround for C99-style
variable declarations, etc.  That's now broken with CRAM code as is;
we'll do some testing on Windows to see if the MSVC situation has
improved or whether the best answer may be to suggest MinGW instead,
perhaps also providing a MinGW-built binary.