config: add condition flags for 'darwin'

Detect 'darwin' in sys.platform and assume that it means that we're on
Mac OS X.

Define new condition flags 'darwin', 'macos', and 'quartz' for that.

We already refer to the 'macos' condition in the bootstrap moduleset but
so far it had never been defined.

The intention is that 'macos' will be used for expectations about the
software available in the system (and what needs to be bootstrapped) and
that 'quartz' will refer to the window system.  It would theoretically
make sense to disable 'quartz' and enable 'x11' if one wanted to use the
X server on macos, for example.

We additionally add a 'gnu-elf' condition for systems that have
GNU-style binutils (which accept -Bsymbolic, for example).
croco,rsvg: disable -Bsymbolic when unavailable

-Bsymbolic is only available on ELF systems with GNU binutils.

The build system of croco and librsvg will complain if it is not
available, so disable it up front to prevent trouble.

This fixes Mac OS X.
bootstrap: remove vala-bootstrap from meta-module

Remove vala-bootstrap from the list of modules build during 'jhbuild
boostrap' since it will fail (due to not-yet-installed GLib).

After GLib is installed, the user will need to type 'jhbuild bootstrap
vala-bootstrap' in order to get things working properly.

This is not a great solution but it is better than the bootstrap
aborting half way through because of a module that has no chance of
building.
bootstrap: add xorg-modules to bootstrap on macos

libepoxy needs xorg-modules in order to build, so add that to the list
of bootstrap modules for macos.