Activate the -fno-common flag of GCC and Clang

So that problems with multiple definitions of global C variables
are detected early during development (see #9144 and #9176 for examples).

configure

@@ -1,4 +1,60 @@
 #! /bin/sh
+
+if test -e '.git' ; then :
+  if test -z "$ac_read_git_config" ; then :
+    extra_args=$(git config ocaml.configure 2>/dev/null)
+    extended_cache=$(git config ocaml.configure-cache 2>/dev/null)
+    cache_file=
+
+    # If ocaml.configure-cache is set, parse the command-line for the --host
+    # option, in order to determine the name of the cache file.
+    if test -n "$extended_cache" ; then :
+      echo "Detected Git configuration option ocaml.configure-cache set to \
+\"$extended_cache\""
+      dashdash=
+      prev=
+      host=default
+      # The logic here is pretty borrowed from autoconf's
+      for option in $extra_args "$@"
+      do
+        if test -n "$prev" ; then :
+          host=$option
+          continue
+        fi
+
+        case $dashdash$option in
+          --)
+            dashdash=yes ;;
+          -host | --host | --hos | --ho)
+            prev=host ;;
+          -host=* | --host=* | --hos=* | --ho=*)
+            case $option in
+              *=?*) host=$(expr "X$option" : '[^=]*=\(.*\)') ;;
+              *=) host= ;;
+            esac ;;
+        esac
+      done
+      cache_file="`dirname "$0"`/$extended_cache/ocaml-$host.cache"
+    fi
+
+    # If either option has a value, re-invoke configure
+    if test -n "$extra_args$cache_file" ; then :
+      echo "Detected Git configuration option ocaml.configure set to \
+\"$extra_args\""
+      # Too much effort to get the echo to show appropriate quoting - the
+      # invocation itself intentionally quotes $0 and passes $@ exactly as given
+      # but allows a single expansion of ocaml.configure
+      if test -n "$cache_file" ; then :
+        echo "Re-running $0 $extra_args --cache-file \"$cache_file\" $@"
+        ac_read_git_config=true exec "$0" $extra_args \
+                                          --cache-file "$cache_file" "$@"
+      else
+        echo "Re-running $0 $extra_args $@"
+        ac_read_git_config=true exec "$0" $extra_args "$@"
+      fi
+    fi
+  fi
+fi
 # Guess values for system-dependent variables and create Makefiles.
 # Generated by GNU Autoconf 2.69 for OCaml 4.11.0+dev0-2019-10-18.
 #
@@ -12442,7 +12498,7 @@ case $host in #(
     case $ocaml_cv_cc_vendor in #(
   clang-*) :
     common_cflags="-O2 -fno-strict-aliasing -fwrapv";
-      internal_cflags="$gcc_warnings" ;; #(
+      internal_cflags="$gcc_warnings -fno-common" ;; #(
   gcc-012-*) :
     # Some versions known to miscompile OCaml, e,g, 2.7.2.1, some 2.96.
       # Plus: C99 support unknown.
@@ -12465,7 +12521,7 @@ $as_echo "$as_me: WARNING: Consider using GCC version 4.2 or above." >&2;};
       internal_cflags="$gcc_warnings" ;; #(
   gcc-*) :
     common_cflags="-O2 -fno-strict-aliasing -fwrapv";
-      internal_cflags="$gcc_warnings" ;; #(
+      internal_cflags="$gcc_warnings -fno-common" ;; #(
   msvc-*) :
     common_cflags="-nologo -O2 -Gy- -MD"
       common_cppflags="-D_CRT_SECURE_NO_DEPRECATE"

configure.ac

@@ -558,7 +558,7 @@ AS_CASE([$host],
   [AS_CASE([$ocaml_cv_cc_vendor],
     [clang-*],
       [common_cflags="-O2 -fno-strict-aliasing -fwrapv";
-      internal_cflags="$gcc_warnings"],
+      internal_cflags="$gcc_warnings -fno-common"],
     [gcc-[012]-*],
       # Some versions known to miscompile OCaml, e,g, 2.7.2.1, some 2.96.
       # Plus: C99 support unknown.
@@ -578,7 +578,7 @@ AS_CASE([$host],
       internal_cflags="$gcc_warnings"],
     [gcc-*],
       [common_cflags="-O2 -fno-strict-aliasing -fwrapv";
-      internal_cflags="$gcc_warnings"],
+      internal_cflags="$gcc_warnings -fno-common"],
     [msvc-*],
       [common_cflags="-nologo -O2 -Gy- -MD"
       common_cppflags="-D_CRT_SECURE_NO_DEPRECATE"