build: Add -Wno-pedantic flag to compiler arguments
We do not support building with -Wpedantic due pervasively casting function pointers through (void*) — see https://wiki.gnome.org/Projects/GLib/CompilerRequirements#Function_pointer_conversions.
Signed-off-by: Philip Withnall withnall@endlessm.com
Helps #1662 (closed)