Fix MS-Windows build broken by "Install C source code"

* nt/epaths.nt (PATH_EMACS_SOURCE): Add definition.

nt/epaths.nt

@@ -83,5 +83,9 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 /* Where Emacs should store game score files.  */
 #define PATH_GAME "%emacs_dir%/var/games/emacs"
 
+/* Where Emacs should look for its own installed source code,
+   or the empty string if the source code is not installed.  */
+#define PATH_EMACS_SOURCE "%emacs_dir%/share/emacs/@VER@"
+
 /* Where Emacs should look for the application default file. */
 #define PATH_X_DEFAULTS ""