Bump meson to 0.58.1 and some fixes for Fedora 34

gobject-introspection: Backport patch for Python 3.9 compat

Needed on Fedora 34.

srt.recipe: Bump to 1.4.3, fixes build on Fedora 34

meson: Bump to 0.58.1

glib-networking: Update options and backport c_std=c99

C11 is only supported by very recent MSVC versions, and
glib-networking doesn't even use C11.