Unbreak build following rev. 282726

(Makefile.inc1): add dependency of xinstall on libmd to
avoid failure of parallel bootstrap.

(lib/libmd/*.h): do not redefine symbols if already
defined as macros (libcrypt uses the same sources internally,
redefining symbols with a prefix of its own).

Fixes build failures caused by previous change to libmd.

Reported by: ian
Pointy hat to: thomas