Bump compiler requirement and coding style to C99
Looks like people are keen to
- declare variables inline
- use advanced/nicer struct initialisers
- use C++-style
GLib made C99 (or at least the subset newer MSVC versions support) a requirement with version 2.64 (we are about to bump our GLib req to 2.56).
We now have pretty good CI coverage, esp. for MSVC, and also bumped the MSVC requirement to drop support for ancient versions.
So there isn't really any technical reason not to officially bump our requirement and also change our coding style.
This shouldn't pose any problems with other compilers such as ICC either.