[Midnightbsd-cvs] mports: mports/Mk: Implement new USE_CSTD flag that ads --std=value to

[laffer1 at midnightbsd.org]

Log Message:
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Implement new USE_CSTD flag that ads --std=value to CFLAGS.  This allows us to force a C standard like c89 on a port in the event we (or the user) want to use alternate compilers.

If we decide to go with llvm + clang someday or a newer GCC this will be quite useful.

Modified Files:
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    mports/Mk:
        bsd.mport.mk (r1.153 -> r1.154)
        (http://cvsweb.midnightbsd.org/mports/Mk/bsd.mport.mk?r1=1.153&r2=1.154)
    mports/Mk/docs:
        bsd.mport.mk.pod (r1.11 -> r1.12)
        (http://cvsweb.midnightbsd.org/mports/Mk/docs/bsd.mport.mk.pod?r1=1.11&r2=1.12)