Set LANG to a reasonable default (C)

Set LANG=C before calling 'rpmbuild' to avoid rpmbuild failing on
the translated date string in the changelog.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #306
Add VPATH and remove $(top_srcdir)/... to make sure it's possible to
do the build in a different directory and not clutter the source with
build results (such as Makefiles, *.o and *.a files etc).

Closes: #1082