diff --git a/platform/unix/gen-makefile b/platform/unix/gen-makefile
index e75836f01..4824822ca 100644
--- a/platform/unix/gen-makefile
+++ b/platform/unix/gen-makefile
@@ -3,7 +3,8 @@ echo Generating src/Makefile.am ...
 cd src
 inc_current=.
 inc_modules="$inc_current/modules"
-echo "AM_CPPFLAGS = -I$inc_current -I$inc_modules -I/usr/include/AL -I/usr/include/freetype2  \$(INCLUDE_LUA) -I/usr/include/SDL \$(FILE_OFFSET)
+inc_box2d="$inc_current/libraries/Box2D"
+echo "AM_CPPFLAGS = -I$inc_current -I$inc_modules -I$inc_box2d -I/usr/include/AL -I/usr/include/freetype2  \$(INCLUDE_LUA) -I/usr/include/SDL \$(FILE_OFFSET)
 AUTOMAKE_OPTIONS = subdir-objects
 DEFAULT_INCLUDES =
 SUBDIRS =