When $USE_HOST or $EXTRAFLAGS are empty, we pass (due to quoting) an
empty string as a parameter to ./configure, which then believes we want
to use a deprecated syntax for specifing a host or a target and yells at us:
> configure: WARNING: you should use --build, --host, --target
The fixes are:
- $EXTRAFLAGS could contain multiple flags and should not be quoted at all.
- We can get rid of $USE_HOST by specifying --host="$HOST" directly.
