Build: Use Nmake-Jumpstart

[xparq]

The Jumpstart makefile got extensively updated, too, so needs merging upstream!

[xparq]

Jesus F Christ, NMAKE + CMD again! :-o :-/ :-(

I've lost almost a day with crap like some "nondescript" unable to execute C:\Program, failing only on the GitHub runner... Could be anywhere in that huge, complex makefile, but finally narrowed down to:

MAKE_CMD=$(MAKE) /nologo /$(MAKEFLAGS) /f $(THIS_MAKEFILE) DEBUG=$(DEBUG) CRT=$(CRT)

...(and in an inline CMD script)...
set "_make_=cmd /c $(MAKE_CMD)"
...
set "_dir_=!_dir_:$(src_dir_abspath)\=!"
...
for %%x in ($(obj_source_exts)) do (
	if exist "%%d\*.%%x" !_make_! /c compiling "DIR=!_dir_!" SRC_EXT_=%%x $(custom_build_options)
	if errorlevel 1 exit -1
)

Oh, no, it's not because of !_make_! not being quoted, no... That was of course the first thing I tried, and that failed, too (I think with the same error, or with something even more hopeless; anyway, it was for a reason I had it unquoted to begin with). Also, an almost identical make command was successfully invoked in the same scriptlet, before the failing one...

Turns out, it was because "DIR=!_dir_!" was quoted!... :-o And it confused (the already by default pretty confused) CMD...
Finally, still in disbelief, but in the height of utter desperation, I reluctantly removed those quotes, as a last-ditch effort (before migrating to gnumake entirely -- which I should've done long ago... :) ), and that apparently fixed it...

(-> https://github.com/xparq/Space_Test, I remember you, with PTSD!...)