[build2] Freezing with MSYS2

[David Cohoe]

Hello,

I just recently installed build2 (0.11) on MSYS2, and it freezes when
run.  By running it with --verbose, I figured out that it gets stuck
on "g++ -D_DEBUG -ggdb3 -x c++ -E -".  I'm guessing that build2
doesn't know how to interact with MSYS2 that well?  I'm using the
MinGW64 prompt if it matters.  I'm a bit surprised that it gets stuck
here, because I built build2 itself in the same terminal.