This simplifies the check in Makefile.PL, and means that "I'm building in the
core" is passed down to build scripts invoked from the Makefile.
# another process has half-written.
$ENV{PERL5LIB}
= join $Config{path_sep}, (map {"$up/$_"} @toolchain), $lib_dir;
+ $ENV{PERL_CORE} = 1;
unless (chdir "$ext_dir") {
warn "Cannot cd to $ext_dir: $!";