Simplify dirprefix macro

Removing subst calls makes the macro less complex and faster, the only
drawback is that some output may contain multiple consecutive '/' which looks
a bit odd, but it has no effect on the functionality.
Remove configurable ifdef's

BOB.BUILD_TEST was for making it possible to always build both tests and all
targets at the same time. But it should be equally simple to run just 'make
test' to build everything. If a developer *requires* this, an alias can be
used.

BOB.DEBUG was only a debugging feature to find empty makerules.mk files,
i.e. files containing no targets. These are typically "binding" files only
specifying additional sub-modules. Maybe useful to investigate slow
performance. (To many developers are creating a lot of directories for no
apparent reason some times).