Fix env. var. naming for cmake instructions.

As weird as it sounds but the proper var. to set is PNG_PNG_INCLUDE_DIR instead
of PNG_INCLUDE_DIR. The latter is overwritten by FindPNG and actually obsolete
(see https://github.com/Kitware/CMake/blob/master/Modules/FindPNG.cmake).

modules/doc/install.rst

@@ -205,7 +205,7 @@ can influence it.
     files directly)
   * `PNG_LIBRARY` defines the location of the libpng library (file name starting
     with `libpng`)
-  * `PNG_INCLUDE_DIR` defines the include folder of libpng (contains include
+  * `PNG_PNG_INCLUDE_DIR` defines the include folder of libpng (contains include
     files directly)
   * `ZLIB_LIBRARY` defines the location of the zlib library (file name starting
     with `libz`)

modules/gfx/src/CMakeLists.txt

@@ -210,7 +210,7 @@ module(NAME gfx SOURCES ${OST_GFX_SOURCES} ${OST_GFX_MAP_SOURCES}
                ${OST_GFX_HEADERS} ${OST_GFX_MAP_HEADERS}
        DEPENDS_ON ${OST_GFX_DEPENDENCIES})
 
-include_directories(${PNG_INCLUDE_DIR} ${OPENGL_INCLUDE_DIR})
+include_directories(${PNG_INCLUDE_DIRS} ${OPENGL_INCLUDE_DIR})
 
 # link against OpenGL and PNG libraries
 target_link_libraries(ost_gfx ${OPENGL_LIBRARIES} ${PNG_LIBRARIES})