Makefile: use "make -C build" idiom

Change an explicit ``cd build && make`` into the more usual
``${MAKE} -C build`` style of invoking make in a subdirectory. This
should mean that ``make -jN`` from the top-level Makefile should work.

Closes #162.
Merge pull request #163 from rjw57/issue-162

Makefile: use "make -C build" idiom