Fix incremental builds involving non-root users with read-only source files.

Makefiles should not assume that source files can be overwritten. This is the
common case for Perforce source trees.

This is a followup commit to r211243 in the same vein.

MFC after: 1 month
Sponsored by: Spectra Logic
MFSpectraBSD: r1036319 on 2014/01/29, r1046711 on 2014/03/06