make Configure abort if both -Duselongdouble and -Dusequadmath are requested

See [perl #126203]

diff --git a/Configure b/Configure
index a6fd51f..d634cdb 100755 (executable)
--- a/Configure
+++ b/Configure
@@ -5241,6 +5241,20 @@ case "$usequadmath" in
 *)                    usequadmath="$undef"  ;;
 esac
 
+: Fail if both uselongdouble and usequadmath are requested
+case "$usequadmath:$uselongdouble" in
+define:define)
+       $cat <<EOM >&4
+
+*** You requested the use of the quadmath library and use
+*** of long doubles.
+***
+*** Please select one or the other.
+EOM
+       exit 1
+        ;;
+esac
+
 : Looking for optional libraries
 echo " "
 echo "Checking for optional libraries..." >&4