This is similar to what was done in
eea4e2ff0a (where it was added to
STAGE*_CXXFLAGS), but this adds the flag to the CXXFLAGS and
BOOT_CXXFLAGS variables too (as it's already added to CFLAGS and
BOOT_CFLAGS).
2021-04-09 Martin Storsjö <martin@martin.st>
config/ChangeLog:
* mh-mingw: Set __USE_MINGW_ACCESS in missed C++ flags
variables