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
Tag spl-0.6.3

META file and release log updated.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>