meson: set HAVE_ELEMENTARY_WIN32 on windows

this enables windows engine selections and other functionality

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D8728

src/lib/elementary/meson.build

@@ -986,6 +986,11 @@ if get_option('cocoa')
   elementary_deps += ecore_cocoa
 endif
 
+if sys_windows == true
+  config_h.set('HAVE_ELEMENTARY_WIN32', '1')
+  elementary_deps += ecore_win32
+endif
+
 c = configure_file(
   output: 'Elementary_Options.h',
   install : true,