New variable ggtags-global-program

In ggtags-global-build-command, which builds the actual command to be
used to call global(1), the filename is hardcoded as "global".

Add ggtags-global-program variable, defaulting to "global", to allow
user to change the filename (e.g. "/opt/local/bin/global" in MacOSX
using MacPorts).

Signed-off-by: Giuseppe 'ferdy' Miceli <ferdy@ferdy.it>