Rebuilding GNAT for mingw (32bit)

[Jere <jhb.chat@gmail.com>]

So I recently ran into an issue using the FSF GNAT on 32bit mingw.  
It ended up having a bug in GNAT.Serial_Communications, which was 
fixed in later versions of GNAT found in 64 bit mingw.  Unfortunately 
I can't upgrade to the 64bit version and get a later version of GNAT.  
The bug fix was only a few lines, so I figured I could make the change 
to g-sercom.adb in the adainclude directory and hopefully recompile 
GNAT.  When I looked up instructions for this I found:

https://gcc.gnu.org/onlinedocs/gnat_ugn/Rebuilding-the-GNAT-Run-Time-Library.html

However, the GNAT installation for mingw did not have that makefile 
and libgnat.a wasn't in the locations specified by the gnatls -v 
command.  I did find libgnat.a, but what I don't know is how to 
recompile it.  Just using gnatmake outright generates a ton of .o 
and .ali files instead.  I'm not even sure which switches the runtime 
is normally compiled with for that platform.  Anyone have experience 
compiling it for mingw?  For reference, the GNAT version for mingw 
is currently 6.3.0.

Any help would be appreciated.