Log Message:
As suggested by Min Sik Kim (on pkgsrc-wip-review@lists.sf.net):
- Remove malloc.h inclusion statement from wnutil.c
- Change PREFIX to BUILdLINK_PREFIX.{tcl,tk} where appropriate
- Do not override vars set by pkgsrc, but append or just let them be
- Include buildlink3.mk files before defining pre-build