This is so that 'make depend' is not a required build step in these
files.
DPSRCS is overall unneeded. DPSRCS already contains SRCS, so anything
which can safely be in SRCS should be. DPSRCS is mostly just a way to
generate files that should not be linked into the final PROG/LIB. For
headers and grammars it is safe for them to be in SRCS since they will
be excluded during linking and installation.
The only remaining uses of DPSRCS are for generating .c or .o files that
must be built before 'make depend' can run 'mkdep' on the SRCS c files
list. A semi-proper example is in tests/sys/kern/acct/Makefile where a
checked-in .c file has an #include on a generated .c file. The
generated .c file should not be linked into the final PROG though since
it is #include'd. The more proper way here is just to build/link it in
though without DPSRCS. Another example is in sys/modules/linux/Makefile
where a shell script runs to parse a DPSRCS .o file that should not be
linked into the module. Beyond those, the need for DPSRCS is largely
unneeded, redundant, and forces 'make depend' to be ran. Generally,
these Makefiles should avoid the need for DPSRCS and define proper
dependencies for their files as well.
An example of an improper usage and why this matters is in usr.bin/netstat.
nl_defs.h was only in DPSRCS and so was not generated during 'make all',
but only during 'make depend'. The files including it lacked proper
depenencies on it, which forced running 'make depend' to workaround that
bug. The 'make depend' target should mostly be used for incremental build
help, not to produce a working build. This specific example was broken in
the meta build until r287905 since it does not run 'make depend'.
The gnu/lib/libreadline/readline case is fine since bsd.lib.mk has 'OBJS:
SRCS:M*.h' when there is no .depend file.
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