(gl_PREREQ_EUIDACCESS): Don't bother checking for

(gl_PREREQ_EUIDACCESS): Don't bother checking for
unistd.h, as autoconf does this for us.  Check for libgen.h.
Also look for eaccess within -lgen, which is where it is in
Solaris.  Set LIB_EACCESS accordingly.
(gl_FUNC_NONREENTRANT_EUIDACCESS): New macro.
(gl_FUNC_EUIDACCESS): Use AC_CHECK_DECLS_ONCE, not AC_CHECK_DECLS.
(gl_PREREQ_EUIDACCESS): Check for eaccess and setregid decls.
Require AC_HEADER_STAT.