Ensure that variable overrides are passed to recursive make instances
even if no overrides were provided on the command line.
Fix suggested by Rici Lake <ricilake@gmail.com>
* src/main.c (define_makeflags): Add overrides without respect to the
value of command_variables.
* tests/scripts/features/recursion: Add a test.
Fix references to MINGW #define constants.
Remove WINDOWS32 ifdef from sub_proc.h.
Only add variables to the command line for recursion once.
New features in run_make_test: #PWD# and #MAKEPATH# replacements.
Test the multi-variable fix in the recursion regression test.