(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
Add a prefix to all symbols in libmd to avoid incompatibilites
with same-named, but not binary compatible, symbols from libcrypto.
Also introduce Weak aliases to avoid the need to rebuild dependent
binaries and a major version bump.
PR: 199119
Differential Revision: D2216
Reviewed by: roberto, delphij
MFC after: 2 weeks