configure: bump min required GCC to 7.4.0

configure: bump min required GCC to 7.4.0

Several distros have been dropped since the last time we bumped the
minimum required GCC version.

Per repology, currently shipping versions are:

                 RHEL-8: 8.3.1
          Debian Buster: 8.3.0
     openSUSE Leap 15.2: 7.5.0
       Ubuntu LTS 18.04: 7.5.0
       Ubuntu LTS 20.04: 9.3.0
                FreeBSD: 10.3.0
              Fedora 33: 9.2.0
              Fedora 34: 11.0.1
                OpenBSD: 8.4.0
         macOS HomeBrew: 11.1.0

With this list Ubuntu LTS 18.04 / openSUSE Leap 15.2 are the
constraint at 7.5.0.

When QEMU bumped GCC to 7.5.0, however, it was reported that
this is a problem for NetBSD which still ships 7.4.0.

NetBSD is not an officially targetted platform for libvirt.
Given that QEMU saw complaints about this and the feature
difference between GCC 7.4.0 and 7.5.0 is minor, I'm being
friendly and sticking 7.4.0.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>