configure: Loosen GCC requirement from 7.5.0 to 7.4.0

configure: Loosen GCC requirement from 7.5.0 to 7.4.0

As discussed in issue 614, we're shipping GCC 7.4.0 as the
system compiler in NetBSD 9, the most recent stable branch,
and are still actively interested in QEMU on this platform.

The differences between GCC 7.5.0 and 7.4.0 are trivial.

Signed-off-by: Nia Alarie <nia@NetBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <YVcpe79I0rly1HJh@homeworld.netbsd.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>