Makefile should build and install the dev subdirectory if needed
Makefile should build and install the dev subdirectory if needed

Summary: Fixes T113.

Test Plan:
Rebuild and install dhcpcd on a system which uses udev and had dhcpcd-6 installed previously.
Test dhcpcd runs without error.

Reviewers: fjaell

Reviewed By: fjaell

Maniphest Tasks: T113

Differential Revision: https://dev.marples.name/D105

rsmarples/dhcpcd,rsmarples/dhcpcd,rsmarples/dhcpcd