[perl #128564] increase parallization for GNU make builds on Win32

# New Ticket Created by  Tony Cook 
# Please include the string:  [perl #128564]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=128564 >


The attached patch improves parallization of perl build on Win32 when using GNU make.

It builds on top of my patch in 128438.

Build times, before and after:

gmake -j6   :  219  128
gmake -j2   :  266  232
gmake       :  321  346

Single job builds take 8% longer, but parallel builds improve significantly.

Tony

• [perl #128564] increase parallization for GNU make builds on Win32 by Tony Cook