Fix --no-nls (3/5), prefer #if in gtk2_ardour

This is for consistency with system-wide gettext.h which is used
by some 3rd party libs. system-wide gettext uses `#if ENABLE_NLS`,
not #ifdef

gtk2_ardour/bundle_env_cocoa.cc

@@ -108,7 +108,7 @@ fixup_bundle_environment (int argc, char* argv[], string & localedir)
 
 	bundle_dir = Glib::path_get_dirname (exec_dir);
 
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 	if (!ARDOUR::translations_are_enabled ()) {
 		localedir = "/this/cannot/exist";
 	} else {

gtk2_ardour/bundle_env_linux.cc

@@ -67,7 +67,7 @@ fixup_bundle_environment (int /*argc*/, char* argv[], string & localedir)
 	}
 #endif
 
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 	if (!ARDOUR::translations_are_enabled ()) {
 		localedir = "/this/cannot/exist";
 	} else {

gtk2_ardour/gettext.h

@@ -20,7 +20,7 @@
 #define _LIBGETTEXT_H 1
 
 /* NLS can be disabled through the configure --disable-nls option.  */
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 
 /* Get declarations of GNU message catalog functions.  */
 # include <libintl.h>

gtk2_ardour/main.cc

@@ -311,7 +311,7 @@ int main (int argc, char *argv[])
 	fftwf_make_planner_thread_safe ();
 #endif
 
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 	/* initialize C locale to user preference */
 	if (ARDOUR::translations_are_enabled ()) {
 		setlocale (LC_ALL, "");
@@ -327,7 +327,7 @@ int main (int argc, char *argv[])
 	windows_vst_gui_init (&argc, &argv);
 #endif
 
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 	cerr << "bind txt domain [" << PACKAGE << "] to " << localedir << endl;
 
 	(void) bindtextdomain (PACKAGE, localedir.c_str());

gtk2_ardour/rc_option_editor.cc

@@ -2280,7 +2280,7 @@ RCOptionEditor::RCOptionEditor ()
 		     ));
 
 
-#ifdef ENABLE_NLS
+#if ENABLE_NLS
 
 	add_option (_("General/Translation"), new OptionEditorHeading (_("Internationalization")));