Fix "make distcheck" when TMPDIR isn't set
Fix "make distcheck" when TMPDIR isn't set

Otherwise, we get this:
 GIT_DIR=./.git git archive --prefix=tinycbor-distcheck/ --format=tar HEAD | tar -xf - -C $TMPDIR
 tar: option requires an argument -- 'C'

Signed-off-by: Thiago Macieira <0ed1648d92ca0373297a182b1dba1b974ad10d88@intel.com>

intel/tinycbor,thiagomacieira/tinycbor,thiagomacieira/tinycbor,thiagomacieira/tinycbor,01org/tinycbor,thiagomacieira/tinycbor,intel/tinycbor,01org/tinycbor,01org/tinycbor,intel/tinycbor,intel/tinycbor