build: install *.mo into the "standard" directory

Change POROJECT_NAME to 'nvim', and use it as the gettext
domain name. The *.mo files, previously installed as
$runtime/lang/xx/LC_MESSAGES/nvim.mo, are now installed as
$prefix/locale/xx/LC_MESSAGES/nvim.mo.

src/nvim/mbyte.c

@@ -571,8 +571,8 @@ char_u * mb_init(void)
 #ifdef HAVE_WORKING_LIBINTL
   /* GNU gettext 0.10.37 supports this feature: set the codeset used for
    * translated messages independently from the current locale. */
-  (void)bind_textdomain_codeset(VIMPACKAGE,
-      enc_utf8 ? "utf-8" : (char *)p_enc);
+  (void)bind_textdomain_codeset(PROJECT_NAME,
+                                enc_utf8 ? "utf-8" : (char *)p_enc);
 #endif