bpo-45723: Add helper macros and more caching to configure.ac (GH-29429)

bpo-45723: Add helper macros and more caching to configure.ac (GH-29429)

Almost all checks are now cached by AC_CACHE_CHECK().

Common patterns are replaced by helper macros.

Variable names now use naming scheme ``ac_cv_func_$funcname``,
``ac_cv_lib_$library_$funcname``, or ``ac_cv_header_$headername_h``.

``SYS_SELECT_WITH_SYS_TIME`` is no longer used.

``uuid_create`` and ``uuid_enc_be`` are provided by libc on BSD. It is
safe to use ``AC_CHECK_FUNCS`` here.

Caching speeds up ./configure -C from ~ 4s to 2.6s on my system.

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no>