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>

configure

@@ -2065,16 +2065,16 @@ cat > $TMPC << EOF
 #  endif
 # endif
 #elif defined(__GNUC__) && defined(__GNUC_MINOR__)
-# if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 8)
-#  error You need at least GCC v4.8 to compile QEMU
+# if __GNUC__ < 7 || (__GNUC__ == 7 && __GNUC_MINOR__ < 5)
+#  error You need at least GCC v7.5.0 to compile QEMU
 # endif
 #else
 # error You either need GCC or Clang to compiler QEMU
 #endif
 int main (void) { return 0; }
 EOF
 if ! compile_prog "" "" ; then
-    error_exit "You need at least GCC v4.8 or Clang v3.4 (or XCode Clang v5.1)"
+    error_exit "You need at least GCC v7.5 or Clang v3.4 (or XCode Clang v5.1)"
 fi
 
 # Accumulate -Wfoo and -Wno-bar separately.