Make the install-* targets depend on things they are installing

This ensures that we are not installing e.g. an obsolete version of the
man page after the Markdown version is updated.

Note that this greatly benefits from the "skopeo" target being
non-phony, otherwise (make install) would rebuild the binary.

Makefile

@@ -57,11 +57,11 @@ install: install-binary install-docs
 	# TODO(runcom)
 	#install -m 644 completion/bash/skopeo ${BASHINSTALLDIR}/
 
-install-binary:
+install-binary: skopeo
 	install -d -m 0755 ${INSTALLDIR}
 	install -m 755 skopeo ${INSTALLDIR}
 
-install-docs:
+install-docs: docs/skopeo.1
 	install -d -m 0755 ${MANINSTALLDIR}/man1
 	install -m 644 docs/skopeo.1 ${MANINSTALLDIR}/man1/