>Thoughts? Comments? 

>+.if !defined(_SRC_TOP_)
>+# Find Makefile.build
>+_SRC_TOP_!=    MF="${.CURDIR}"; \
>+               while `/usr/bin/true`; \
>+               do \
>+                       echo $${MF} >&2;  \
>+                       if [ -f "$${MF}/Makefile.build" ]; then \
>+                               echo "$${MF}/"; \
>+                               break; \
>+                       fi; \
>+                       dir1=`cd $${MF}; pwd`; \
>+                       dir2=`cd $${MF}/..; pwd`; \
>+                       if [ "$$dir1" = "$$dir2" ]; then \
>+                               echo ""; \
>+                               break; \
>+                       else \
>+                               MF="$${MF}/.."; \
>+                       fi; \
>+               done

This is more complex than necessary.  "while :" is all you need 
for an infinite loop btw.  Anyway, either simply use `dirname` on 
your current guess or my preference:

_SRC_TOP_!= cd ${.CURDIR}; while :; do \
	    here=`pwd`; \
	    [ -f Makefile.build  ] && { echo $$here; break; }; \
	    case $$here in /) break;; esac; \
	    cd ..; \
	done 

>+MAKE+= _SRC_TOP_=${_SRC_TOP_:Q}

Also I'm not at all keen on appending to MAKE.
Use .MAKEFLAGS or 
.MAKEOVERRIDES+= _SRC_TOP_
will do the trick.

Thanks
--sjg