This fixes incremental build of OpenSSH after the recent upgrade.
For example, in secure/lib/libssh, -include ssh_namespace.h is used on
all files. This is not tracked in the .depend file though due to
MKDEP_CFLAGS not including it. The ssh example was broken in r291941
when not using FAST_DEPEND due to the .depend bug. FAST_DEPEND was not
affected by this because it generates dependencies at compile time and
thus sees the -include.
This ugly make syntax could be simpler for bmake by using :tW but
fmake-compatible syntax is used since this needs to be MFC'd all the way
to stable/9.
Also add a temporary hack to workaround existing checkouts building
incrementally with a .depend file not having these headers.
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division
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