Add the -d2VolatileMetadata- switch to cl

Visual Studio 2019 and 2022 turn on the generation of volatile metadata
by default, which can cause problems for flexlink which doesn't know how
to process the .voltbl section properly.

This information has been enabled to aid emulation of AMD64 on ARM64, so
for now risk slower emulated performance pending direct compilation to
ARM64 on Windows anyway.

Changes

@@ -369,6 +369,11 @@ OCaml 4.14.0
   (Sébastien Hinderer, review by David Allsopp, Daniel Bünzli, Xavier Leroy
   and Gabriel Scherer)
 
+- #10797: Compile with -d2VolatileMetadata- on supporting versions of Visual
+  Studio. This suppresses the addition of .voltbl sections and eliminates
+  linking errors in systhreads.
+  (David Allsopp, review by ???)
+
 ### Bug fixes:
 
 - #9214, #10709: Wrong unmarshaling of function pointers in debugger mode.

aclocal.m4

@@ -129,6 +129,19 @@ AC_DEFUN([OCAML_CC_HAS_DEBUG_PREFIX_MAP], [
   CFLAGS="$saved_CFLAGS"
 ])
 
+AC_DEFUN([OCAML_CL_HAS_VOLATILE_METADATA], [
+  AC_MSG_CHECKING([whether the C compiler supports -d2VolatileMetadata-])
+  saved_CFLAGS="$CFLAGS"
+  CFLAGS="-d2VolatileMetadata- $CFLAGS"
+  AC_COMPILE_IFELSE(
+    [AC_LANG_SOURCE([int main() { return 0; }])],
+    [cl_has_volatile_metadata=true
+    AC_MSG_RESULT([yes])],
+    [cl_has_volatile_metadata=false
+    AC_MSG_RESULT([no])])
+  CFLAGS="$saved_CFLAGS"
+])
+
 # Save C compiler related variables
 AC_DEFUN([OCAML_CC_SAVE_VARIABLES], [
   saved_CC="$CC"

configure.ac

@@ -676,6 +676,9 @@ AS_CASE([$host],
       [common_cflags="-nologo -O2 -Gy- -MD $cc_warnings"
       common_cppflags="-D_CRT_SECURE_NO_DEPRECATE"
       internal_cppflags='-DUNICODE -D_UNICODE'
+      OCAML_CL_HAS_VOLATILE_METADATA
+      AS_IF([test "x$cl_has_volatile_metadata" = "xtrue"],
+            [internal_cflags='-d2VolatileMetadata-'])
       internal_cppflags="$internal_cppflags -DWINDOWS_UNICODE="
       internal_cppflags="${internal_cppflags}\$(WINDOWS_UNICODE)"],
     [xlc-*],