gtkdoc-scangobj: support double quotes in flags and multiple flags parameters

Permit to pass cflags (mostly) containing double quotes to gcc, this is something that could happen in case you want to reuse the same cflags you use for the main library without defining new ones.

Also add support for passing --cflags and --ldflags arguments multiple times, so that the values can be overridden by build systems. For example this also will allow to fix issues such as https://github.com/mesonbuild/meson/issues/2901 in environments where a fixed meson (coming soon) isn't available.