> Try defining _SRC_TOP_ - to a nullstring if you don't have the new tools,
> or to the directory containing build.sh if you do.

It makes a difference. Though system and user times are still more than
twice as much as they were before.

Now, _SRC_TOP_ is a static value for a location of a given source
tree, so there's absolutely no point in re-computing it in every
invocation of make during a subtree walk.

I'd rather see it move to build.sh from where it can be added as a make
variable on the command line. This would have the added advantage that
just typing `make' in any subdirectory of the source tree will use
the installed toolchain instead of defaulting to the one in src/tools.