build-sys: do no support modules on Windows

Our module system does not support Windows, because it relies on
resolving symbols from the main executable.

If there is enough interest in supporting modules on Windows, we could
generate an import library for the executable and link with it:
https://stackoverflow.com/questions/15454968/dll-plugin-that-uses-functions-defined-in-the-main-executable

However, there is a small chicken egg problem, since the executable
link and exports extra symbols needed by the library...
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190718120413.27678-1-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -1752,7 +1752,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   guest-agent     build the QEMU Guest Agent
   guest-agent-msi build guest agent Windows MSI installation package
   pie             Position Independent Executables
-  modules         modules support
+  modules         modules support (non-Windows)
   debug-tcg       TCG debugging (default is disabled)
   debug-info      debugging information
   sparse          sparse checker
@@ -2007,6 +2007,11 @@ else
   QEMU_CFLAGS="$QEMU_CFLAGS -Wno-missing-braces"
 fi
 
+# Our module code doesn't support Windows
+if test "$modules" = "yes" && test "$mingw32" = "yes" ; then
+  error_exit "Modules are not available for Windows"
+fi
+
 # Static linking is not possible with modules or PIE
 if test "$static" = "yes" ; then
   if test "$modules" = "yes" ; then