Fix Win32 build problem caused by change 23757

Fix Win32 build problem caused by change 23757

Change 23757 added a glob() call to win32/FindExt.pm.  That code is
run by miniperl.exe when making the ..\config.sh target, but
miniperl.exe is built with -D PERL_EXTERNAL_GLOB so it requires
perlglob.exe to do the glob().  perlglob.exe has been built, but is
in the top-level of the source tree so is not found when miniperl.exe
is executed from within the win32/ sub-directory.

This was causing smokes to fail on t/lib/commonsense.t because the
"extensions" field in lib/Config_heavy.pl only contained
"threads/shared" as a result of the glob() not finding anything.

Manual builds had been working fine for me because I had an installed
perl in my PATH so perlglob.exe was being found there instead!

p4raw-id: //depot/perl@23785