build: FAST_DEPEND default (kernel first, then world)

[Bryan Drewery]

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On 1/27/16 11:27 AM, Bryan Drewery wrote:
> I have been asked to also remove the old mkdep version of 'make
> depend'.

I should note some of the bugs with the old 'mkdep' method that are
fixed by the FAST_DEPEND method, for world.

1. CXXFLAGS/CFLAGS are not passed properly from Makefile.inc1 for
external toolchain with gcc. A DEPFLAGS hack was added to address
this. The problem being that we pass CFLAGS via CC="${CC} ${CFLAGS}"
from Makefile.inc1 rather than a CFLAGS_APPEND that mkdep can pick up.
2. CFLAGS=-include was ignored in mkdep until r294370.
3. Avoiding ccache for mkdep involves annoying hacks that broke
someone's build (unfixed)
4. ccache can't benefit from mkdep
5. Similar to -include, we only pass certain flags to mkdep which is a
maintenance problem.
6. It hides dependency problems due to 'requiring' running 'make
depend' before build. A goal with FAST_DEPEND is to not need this,
which has proven to be fine.  DPSRCS was widely abused and fixed. It
should not even exist really, it's not needed.

- -- 
Regards,
Bryan Drewery
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