Adapt Configure to GCC version 10

Adapt Configure to GCC version 10

I got a notice from Jeff Law <law@redhat.com>:

    Your particular package fails its testsuite. This was ultimately
    tracked down to a Configure problem. The perl configure script treated
    gcc-10 as gcc-1 and turned on -fpcc-struct-return. This is an ABI
    changing flag and caused Perl to not be able to interact properly with
    the dbm libraries on the system leading to a segfault.

His proposed patch corrected only this one instance of the version
mismatch. Reading the Configure script revealed more issues. This
patch fixes all of them I found.

Please note I do not have GCC 10 available, I tested it by faking the version
with:

--- a/Configure
+++ b/Configure
@@ -4672,7 +4672,7 @@ $cat >try.c <<EOM
 int main() {
 #if defined(__GNUC__) && !defined(__INTEL_COMPILER)
 #ifdef __VERSION__
-       printf("%s\n", __VERSION__);
+       printf("%s\n", "10.0.0");
 #else
        printf("%s\n", "1");
 #endif