Assumes GNU cp is in path; a bug should be filed upstream to not use
GNU-specific flags or configure should detect GNU cp and require it.

Patch itself is not appropriate for upstream.

XXX file upstream bug

--- p11-kit-0.23.1/build/gtk-doc.make	2014-11-12 02:58:50.000000000 -0800
+++ p11-kit-0.23.1/build/gtk-doc.make	2015-05-27 14:23:15.544773257 -0700
@@ -82,7 +82,7 @@
 	    if test "x$$files" != "x" ; then \
 	        for file in $$files ; do \
 	            test -f $(abs_srcdir)/$$file && \
-	                cp -pu $(abs_srcdir)/$$file $(abs_builddir)/ || true; \
+	                gcp -pu $(abs_srcdir)/$$file $(abs_builddir)/ || true; \
 	        done; \
 	    fi; \
 	fi