Define PROG, for newer versions of skalibs

Needed to build from our source tarball on some Gentoo systems.

Closes #239.
Closes #254.

src/frontend/mosh-client.cc

@@ -30,6 +30,11 @@
 #include <curses.h>
 #include <term.h>
 
+/* For newer skalibs */
+extern "C" {
+  const char *PROG = "mosh-client";
+}
+
 void usage( const char *argv0 ) {
   fprintf( stderr, "mosh-client (%s)\n", PACKAGE_STRING );
   fprintf( stderr, "Copyright 2012 Keith Winstein <mosh-devel@mit.edu>\n" );