+ use _WIN32/_WIN64 in preference to __MINGW32__/__MINGW64__ symbols
to simplify building with MSVC, since the former are defined in both
compiler configurations (report by Ali Abdulkadir).
+ further improvements to configure-checks from work on dialog, i.e.,
updated CF_ADD_INCDIR, CF_FIND_LINKAGE, CF_GCC_WARNINGS,
CF_GNU_SOURCE, CF_LARGEFILE, CF_POSIX_C_SOURCE, CF_SIZECHANGE, and
CF_TRY_XOPEN_SOURCE.
+ update config.guess, config.sub from
http://git.savannah.gnu.org/cgit/config.git