Raising our compiler requirements for 9.6

Hi,

During the 9.5 cycle, and earlier, the topic of increasing our minimum
bar for compilers came up a bunch of times. Specifically whether we
still should continue to use C90 as a baseline.

I think the time has come to rely at least on some newer features.

At the very least I think we should start to rely on 'static inline's
working. There is not, and hasn't been for a while, any buildfarm animal
that does not support it and we go through some ugly lengths to avoid
relying on inline functions in headers.  It's a feature that has been
there in most compilers long before C99.

My feeling is that we shouldn't go the full way to C99 because there's
still common compilers without a complete coverage. But individual
features are fine.

The list of features, in the order of perceived importance, that might
be worthwhile thinking about are:
* static inline
* variadic macros
* designated initializers (e.g. somestruct foo = { .bar = 3 } )
* // style comments (I don't care, but it comes up often enough ...)

Others might have different items. I think we should *not* decide on all
of them at once. We should pick items that are supported everywhere and
uncontroversial and do those first.

Greetings,

Andres Freund