[Vala] Conditional compilation: Passing #ifdefs through to generated C code

[Michael Terry <mike mterry name>]

I'd like to know more about how to use C preprocessor commands in
Vala.  I know that Vala allows some sort of conditional compilation
via [Conditional], but I'm interested in a particular use case that it
doesn't seem to really help with.

Specifically, I have a GTK+ app, and I want to use a bit of code only
if the compile-time GTK+ version is high enough.  So something like:

#if GTK_CHECK_VERSION(2, 4, 0)
    GtkFileChooserAction action;
    ...
#endif

I can have vala conditionally use a bit of code, but the conditional
is resolved at Vala-compile-time, not C-compile-time.  I need
'passthrough' conditionals that show up in the generated C code, since
my software distributes the C code in the release tarball.  The
typical user just compiles the C.

-mt