Detecting building under the core

As far as I can tell, when the core is building extentions, it invokes the
Makefile.PL with PERL_CORE=1 on the command line, and from this MakeMaker
sets PERL_CORE=1 inside the Makefile. In turn, MakeMaker contains rules to
extract files from .PL files.

However, it seems that the information about "building under the core" isn't
passed through in any way. Am I right? If so, what's the cleanest way to pass
into a .PL file that we're running in a build under the core source directory?

I was rather hoping to avoid heuristics, as both "I can't find the installed
libraries" and "there is a file "perl.h" $n directory levels up" could go
wrong.

Nicholas Clark