configure: bump min required GCC to 7.5.0

configure: bump min required GCC to 7.5.0

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

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210514120415.1368922-12-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>