Install a public "gnetworking.h" header that can be used to include
the relevant OS-dependent networking headers. This does not really
abstract away unix-vs-windows however; error codes, in particular,
are incompatible.
gnetworkingprivate.h now contains just a few internal URI-related
functions
Also add a g_networking_init() function to gnetworking.h, which can be
used to explicitly initialize OS-level networking, rather than having
that happen as a side-effect of registering GInetAddress.
https://bugzilla.gnome.org/show_bug.cgi?id=623187
_GNU_SOURCE must be defined before including any other (system)
header, so defining it in glib-unix.h (and hoping no one has included
anything else before that) is wrong. And the "#define _USE_GNU"
workaround for this problem in gnetworkingprivate.h is even wronger
(and still prone to failure anyway due to single-include guards).
Fix this by defining _GNU_SOURCE in config.h when building against
glibc. In theory this is bad because new releases of glibc may include
symbols that conflict with glib symbols, which could then cause
compile failures. However, most people only see new releases of glibc
when they upgrade their distro, at which point they also generally get
new releases of gcc, which have new warnings/errors to clean up
anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=649201
