Re: C11 / VS 2019

Ok, now that we have seemingly stabilized the VS 2019 upgrade, here is 
the next patch set to actually raise the compiler requirement to C11.

Viewed from very far away, this just adjusts the existing places that 
say C99 and replaces them with a C11 analogue.

The details are a bit of a longer story.

configure.ac previously used AC_PROG_CC_C99 to activate C99.  But there 
is no AC_PROG_CC_C11 in Autoconf 2.69, because it's too old.  Also, 
post-2.69, the AC_PROG_CC_Cnn macros were deprecated and AC_PROG_CC 
activates the last supported C mode.

So, at this point, we could "just" update the Autoconf version requirement.

But somehow I don't feel like doing that, as it's just another 
non-trivial project to negotiate.  Instead, I just hand-coded some test 
for C11 using some inspiration from later Autoconf versions.  But 
instead of writing an elaborate test program that exercises many 
different features, I kept it simple and just check __STDC_VERSION__, 
which should be good enough in practice.

(If someone later wanted to update the Autoconf version, they could just 
delete that code again.)

In meson.build, there is an existing hand-coded C99 test that I update 
to C11, but again just checking for __STDC_VERSION__.

I also moved the test a bit earlier in meson.build, as a separate patch, 
because between the place where the compiler is first set up and the 
place where we detected the C99 options, if any, there were already some 
tests run that use the compiler.  I don't think this was a big problem 
in practice, but it seems a bit incorrect, so it makes sense to correct it.

Note, we don't use the "official" way to set the C standard in Meson 
using the c_std project option, because that is IMO impossible to use 
correctly (see <https://github.com/mesonbuild/meson/issues/14717>). 
(Well, that is my reason now.  I don't know what the reason was 
previously.)  The disadvantage is that meson will complain like

meson.build:3013: WARNING: Consider using the built-in option for 
language standard version instead of using "/std:c11".

But you will get warnings from meson with MSVC anyway, so, uh, another 
one will not make a significant difference.  (Also, this issue exists 
with the existing C99 detection code as well, except that in practice 
you don't need an option, so the warning does not appear.)

Note that gcc and clang switched to C11 by default a long time ago 
(gcc-5 and clang-3.6), so for most users all these tests won't need to 
do anything.  If you want to test it, you could simulate an older 
default like

./configure CC='gcc -std=c99'

and then the test should decide that it needs to add another option to 
override the C mode.