How to include Multiple headers

[Jason Williams]

When I build a program which contains multiple headers in other path, how 
should I include those headers in GNUmakefile? 

The error throws 
    SurfaceView.h: No such file or directory

The tree structure looks like
    OpenWar/Sources-Debian/ # location of GNUmakefile

    Library/ViewCore/ # location of SurfaceView.h 

My GNUmakefile looks like 

include $(GNUSTEP_MAKEFILES)/common.make
APP_NAME = OpenWar 
OpenWar_HEADERS = ../../Library/ViewCore/SurfaceView.h
OpenWar_OBJC_FILES = main.m OpenWarDocument.m
OpenWar_RESOURCE_FILES =
include $(GNUSTEP_MAKEFILES)/application.make

Complete messages during compilation is as below:

This is gnustep-make 2.6.2. Type 'make print-gnustep-make-help' for help.
Making all for app OpenWar...
 Creating OpenWar.app/....
 Compiling file main.m ...
 Compiling file OpenWarDocument.m ...
In file included from OpenWarDocument.m:5:0:
OpenWarDocument.h:5:24: fatal error: SurfaceView.h: No such file or directory
compilation terminated.
make[3]: *** [obj/OpenWar.obj/OpenWarDocument.m.o] Error 1
make[2]: *** [internal-app-run-compile-submake] Error 2
make[1]: *** [OpenWar.all.app.variables] Error 2
make: *** [internal-all] Error 2