use $< and $@ to avoid duplication

Follow the DRY rule: if the rule's target or dependency change, it's
less error-prone to avoid requiring that it be changed everywhere it's
used.
remove superfluous check from install-markdown

shocco is not required in order for the install-markdown target to be
built.
make shocco a proper dependency of the `install' target
always echo message when shocco has been built

If we run `make install', it should still show that shocco
was rebuilt.
Merge pull request #31 from aspiers/makefile-tweaks

Miscellaneous Makefile improvements