Add a DFLAGS variable for dtrace(1).

[markj]

Summary

This change adds DFLAGS to sys.mk. It can be overridden by programs
or libraries that wish to compile USDT probes with lazyload mode.

This change also adds -C to the default dtrace(1) flags. This causes
dtrace(1) to run the C preprocessor on D scripts; one can also add
-D to DFLAGS to set preprocessor variables.

While in the neighbourhood, correctly sort definitions for CP, CPP and
DTRACE.

[imp]

This is fine, but makes the name space pollution for strict POSIX mode a little worse than it is today.

share/mk/sys.mk

65–70

These don't belong in the POSIX section. But we've kinda fallen off the POSIX bandwagon in other places.

[markj]

In D2204#3, @imp wrote:

This is fine, but makes the name space pollution for strict POSIX mode a little worse than it is today.

I could wrap DTRACE and DFLAGS in a .if !defined(%POSIX), assuming that's what you're referring to by strict POSIX mode.

[markj]

• Use DTRACEFLAGS instead of DFLAGS.

[markj]

Closed by commit rS281258 (authored by @markj).