doc: Fix races in a parallel build.

doc: Fix races in a parallel build.

* configure.ac: Use AC_PROG_MKDIR_P macro.
* doc/Makefile.am (stamp_functions, stamp_enums): Use the MKDIR_P
variable it defines.
(error_codes.texi, algorithms.texi, alerts.texi): Add dependency on
errcodes via a prerequisite, not a make invocation
(DISTCLEANFILES): Register the newly depended upon binaries.

Fixes: <https://gitlab.com/gnutls/gnutls/-/issues/1635>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>