build: clean up extra path separator after DESTDIR

I noticed this admittedly harmless cosmetic thing when I installed ck to /usr/local. But it's trivial to fix.

before:
```
[pierce@plo-air ck]$ make DESTDIR=./pkg install
make -C doc all || exit
mkdir -p ./pkg//usr/local/include || exit
mkdir -p ./pkg//usr/local/lib
mkdir -p ./pkg//usr/local/lib
cp src/libck.so ./pkg//usr/local/lib/libck.so.0.4.4
cp src/libck.a ./pkg//usr/local/lib/libck.a
ln -sf libck.so.0.4.4 ./pkg//usr/local/lib/libck.so
chmod 644 ./pkg//usr/local/lib/libck.a
ln -sf libck.so.0.4.4 ./pkg//usr/local/lib/libck.so.0
chmod 744 ./pkg//usr/local/lib/libck.so.0.4.4	\
	  ./pkg//usr/local/lib/libck.so		\
	  ./pkg//usr/local/lib/libck.so.0
cp /home/pierce/scm/ck/include/*.h ./pkg//usr/local/include || exit
...
```

after:
```
[pierce@plo-air ck]$ make DESTDIR=./pkg install
make -C doc all || exit
mkdir -p ./pkg/usr/local/include || exit
mkdir -p ./pkg/usr/local/lib
mkdir -p ./pkg/usr/local/lib
cp src/libck.so ./pkg/usr/local/lib/libck.so.0.4.4
cp src/libck.a ./pkg/usr/local/lib/libck.a
ln -sf libck.so.0.4.4 ./pkg/usr/local/lib/libck.so
chmod 644 ./pkg/usr/local/lib/libck.a
ln -sf libck.so.0.4.4 ./pkg/usr/local/lib/libck.so.0
chmod 744 ./pkg/usr/local/lib/libck.so.0.4.4	\
	  ./pkg/usr/local/lib/libck.so		\
	  ./pkg/usr/local/lib/libck.so.0
cp /home/pierce/scm/ck/include/*.h ./pkg/usr/local/include || exit
...
```