Create OS X application bundles for PuTTY and pterm.

This commit adds two .plist files, which go in the app bundles; two
.bundle files, which are input to gtk-mac-bundler and explain to it
how to _create_ the bundles; and a piece of manual addition to
Makefile.am that actually runs gtk-mac-bundler after building the
gtkapp.c based binaries and the OSX launcher. The latter is
conditionalised on configuring --with-quartz (unlike the binaries
themselves, which you can build on other platforms too, though they
won't do much that's useful).

Recipe

@@ -192,6 +192,18 @@ install-exec-local:
 endif
 !end
 
+# In automake makefile, build the OS X app bundle, if configured in
+# Quartz mode.
+!begin am
+if HAVE_QUARTZ
+noinst_SCRIPTS = unix/PuTTY.app unix/Pterm.app
+unix/PuTTY.app: unix/putty.bundle puttyapp osxlaunch
+	rm -rf $@ && gtk-mac-bundler $<
+unix/Pterm.app: unix/pterm.bundle ptermapp osxlaunch
+	rm -rf $@ && gtk-mac-bundler $<
+endif
+!end
+
 # Random symbols.
 !begin cygwin vars
 # _WIN32_IE is required to expose identifiers that only make sense on