Makefile.am: Always use C locale for "sort" and "comm".

Otherwise, if the user changes locales between running the "dist-hook-git"
and "distfiles" targets (e.g. in different invocations of "make"), then
the "dist-hook-git" target might falsely report that the distribution is
missing files.

Reported-by: John Darrington <john@darrington.wattle.id.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>