Portability and style fixes:

Portability and style fixes:
  * Actually use the HAVE_<header>_H macros to conditionally include
    system headers.  They've been defined for a long time, but only
    used in a few places.  Now they're used pretty consistently
    throughout.
  * Fill in a lot of missing casts for conversions from void*.
    Although Standard C doesn't require this, some people have been
    trying to use C++ compilers with this code, and they do require it.

Bit-for-bit, the compiled object files are identical, except for
one assert() whose line number changed, so I'm pretty confident I
didn't break anything.  ;-)