RFC: feature tests instead of compiler tests

[Alexander Leidinger]

Hi,

the 106k large patch at
http://www.leidinger.net/FreeBSD/current-patches/compiler_independence.diff
introduces feature tests instead of checks for a particular compiler in
the kernel source. The only checks for a particular compiler are in
cdefs.h now.

As per suggestion from bde I also removed parts of the backward
compatibility to old gcc versions because the rest of the kernel can't
be compiled anymore with those old versions.

Interested parties may look at the cdefs.h part first and suggest some
fixes (e.g. better names for the feature defines).

The patch survives a run through the universe. Various revisions of this
patch run fine on my desktop system since some months.

Bye,
Alexander.

-- 
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http://www.Leidinger.net                       Alexander @ Leidinger.net
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