[tlbuild] MinGW cross-compiling, settings CFLAGS/LDFLAGS etc.

[Karl Berry]

Sorry, I have no advice for cross-compiling.

    so I assume the luatex53 flag
    could go for good?

That's my understanding too. I'll look into it.

    Running ./Build --help leads to errors and breaks the build if another
    build in the same directory is already running. Is there any way to
    avoid this?

1) yes, run ./configure --help instead of ./Build --help.
All Build does is pass on args to configure.

2) It should be possible to run simultaneous Build(s), whether --help or
not, by set the envvars TL_WORKDIR (to override "Work/" as the build
dir), and TL_INSTALL_DEST (to override "inst/" as the install dir). 
I admit I've tried it, but I think those are the two that would matter.
Essentially everything Build does can be overridden with envvars. -k