docs: Stop mentioning outdated C versions in various docs

Link to the toolchain requirements, which are kept up to date, rather
than duplicating them across multiple documents.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

CONTRIBUTING.md

@@ -120,7 +120,7 @@ development tools appropriate for your operating system, including:
  - Meson
  - Ninja
  - Gettext (19.7 or newer)
- - a [C99 compatible compiler](./docs/toolchain-requirements.md)
+ - a [compatible toolchain](./docs/toolchain-requirements.md)
 
 Up-to-date instructions about developing GNOME applications and libraries
 can be found on [the GNOME Developer Center](https://developer.gnome.org).

INSTALL.md

@@ -15,9 +15,8 @@ meson install -C _build                 # install GLib
 Requirements
 ============
 
-GLib requires a C90-compliant (but not necessarily C99-compliant) C
-compiler and libc. On UNIX-like systems, it also assumes compliance
-with at least the original 1990 version of POSIX.
+GLib requires a [basic C toolchain](./docs/toolchain-requirements.md) and
+support for a minimum version of the C standard.
 
 GLib-2.0 requires pkg-config, which is tool for tracking the
 compilation flags needed for libraries. (For each library, a small `.pc`