Add support for OpenPGP on Windows

plugins/gpgme-vala/src/gpgme_helper.vala

@@ -176,6 +176,9 @@ private static uint8[] get_uint8_from_data(Data data) {
 
 private static void initialize() {
     if (!initialized) {
+#if _WIN32
+        set_global_flag("w32-inst-dir", "C://Program Files (x86)//GnuPG//bin");
+#endif
         check_version();
         initialized = true;
     }

plugins/gpgme-vala/vapi/gpgme_public.vapi

@@ -4,6 +4,9 @@ namespace GPG {
 [CCode (cname = "gpgme_check_version")]
 public unowned string check_version(string? required_version = null);
 
+[CCode (cname = "gpgme_set_global_flag")]
+public int set_global_flag(string name, string value);
+
 [Compact]
 [CCode (cname = "struct _gpgme_key", ref_function = "gpgme_key_ref_vapi", unref_function = "gpgme_key_unref_vapi", free_function = "gpgme_key_release")]
 public class Key {