The conversion to multi-byte will be locale-specific, but it's
better than nothing and is only to provide more information in the
error message returned by gz_error().  The conversion has no
effect on what's opened.

Also need to #include <stddef.h> for zlib.h, and need to workaround
the inability to use wide characters in constructed error messages
with zlib's interface.

	modified:   CMakeLists.txt

Add a version variable so it gets added to the pkg-config file.

Using optimized byte swaps reduced portability for no real benefit,
since they are in parts of the code that represent a tiny fraction
of the execution time.  So a simple definition of a byte swap is
now used.