In share/mk/bsd.dep.mk (v1.67), the dependency for .depend is the
following:

     .depend: ${__DPSRCS.d}

where ${__DPSRCS.d} is derived from ${SRCS} if it is defined.
One consequence of this is that dynamically constructed files will
build with 'make all' but will not exist for an earlier 'make
depend'.  Earlier incarnations of this file included a dependency on
${SRCS} for the .depend target.

This behavior was removed in bsd.dep.mk (v1.54) with the following
comment:

     Revision 1.54, Fri Aug 1 00:55:43 2003 UTC (9 months, 1 week ago) by lukem 
     Branch: MAIN 
     Changes since 1.53: +2 -2 lines 

     tweak ${DEPENDSRCS.d} target so that ${SRCS} isn't there.  this fixes the
     "make: out of memory" problem when make dependencies in lib/libc.

     XXX: this is a temporary workaround.  i'm working on cleaning up how
     the DPSRCS stuff is used throughout the tree...

Since that time a lot has changed in the structure of bsd.dep.mk.  I
wonder if it is ok to reintroduce the ${SRCS} dependency, so that
Makefiles with dynamically created source code will be able to make
their dependencies properly.

Cheers,
Brook