On a __GLIBC__ system, we need to use __xpg_strerror_r() -- this is already checked for, so simply one of these checks to reduce code changes from upstream.

Problem pointed out by naddy for FreeBSD -portable. Discussed with millert, thomas adam, and naddy.

This adds the capability to compile got-portable on FreeBSD.

It looks like autoconf 2.69 will not provide XPG strerror_r even if the appropriate macros are set which request the XPG version. Until this is fixed in autoconf and that version is more widely-adopted, this approach will provide what's required. Originally from Stefan Sperling, tweaked by me.

This commit modifies the GoT main branch to be able to compile it under linux.

This is still not thread-safe but at least avoids errors overwriting each other in single-threaded contexts.

got_error_fmt() could eventually replace got_error_path() which has already been used to construct errors with strings that are not actually paths... ok millert@

suggested by and ok millert

$> got rm a got: : No such file or directory becomes $> got rm a got: /path/to/a: No such file or directory

also add got_error_prefix_errno2 and got_error_prefix_errno3 which should hopefully all be merged into a single function with variadic args (but can't alloc mem)