When bootstrapping on FreeBSD we use -DNO_SHARED so this case is already
handled correctly, but on non-FreeBSD we set NO_SHARED to no in
Makefile.boot.pre as not all OSes have static libraries available. As a
result, users of libdwarf fail to link during the cross tools stage due
to the newly-introduced dependency of libdwarf on libz.
This should perhaps be reworked to instead leave NO_SHARED as yes but
have an override (either implicit in bsd.prog.mk, or explicit via a new
variable) to turn off just the use of -static.
MFC after: 1 week
f0a6ab51e6