MFH: r498230

Include stdarg.h in src/cppstring.cpp to fix build with GCC-based
architectures:

/usr/local/include/gtkmm-2.0/glibmm/thread.h: In function 'void Glib::thread_init(GThreadFunctions*)':

Also add USES=dos2unix (to fix patching cppstring.cpp) gnome.

PR: 237040
Submitted by: Piotr Kubaj

Approved by: portmgr (tier-2 blanket)