Fix build with read-only source dir after 83c20b8a2da0

I changed the Makefile to use SRCS instead of LDADD, but since there is
still and absolute path to the source the .o file was created inside the
source directory instead of the build directory.
It would be nice if this was an error/warning by default, but for now just
fix this issue by using .PATH and the base name of the file.

Reported by: cy, peterj