add SHELL variable to Makefile

I'm using Ubuntu 14.04. I had no problem with `make`, but `make install` fails with the following error messages:

```bash
$ make install
(...snip...)

install -D object/libmdbm.so.4 /tmp/install/lib64
#ln -s -f -r /tmp/install/lib64/libmdbm.so.4 /tmp/install/lib64/libmdbm.so
# some systems don't support "ln -r" (relative links), emulate it...
pushd /tmp/install/lib64; ln -s -f -r libmdbm.so.4 libmdbm.so; popd
/bin/sh: 1: pushd: not found
/bin/sh: 1: popd: not found
make[2]: *** [install] Error 127
make[2]: Leaving directory `/home/vagrant/repo/mdbm/src/lib'
make[1]: *** [make-recursive-install] Error 2
make[1]: Leaving directory `/home/vagrant/repo/mdbm/src'
make: *** [make-recursive-install] Error 2
```

`pushd` and `popd` are bash built-in commands. But Ubuntu's default shell is dash, where those commands are not available. To make things work, the environmental variable `SHELL` should be specified in Makefile.

(I'm not familiar with Makefile. Sorry if this pull request is not a good way of setting `SHELL`...)

cf. [GNU make - 5.3.2 Choosing the Shell](http://www.gnu.org/software/make/manual/make.html#Choosing-the-Shell)