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_EUIDACCESS): Use AC_CHECK_DECLS_ONCE, not AC_CHECK_DECLS.
(gl_PREREQ_EUIDACCESS): Check for eaccess and setregid decls.
Require AC_HEADER_STAT.