diff --git a/configure.ac b/configure.ac
index 67ddc17..42f22d5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -69,6 +69,9 @@ AC_CHECK_HEADERS(SDL_image.h,
  [ac_cv_my_have_sdl="yes"],
  [ac_cv_my_have_sdl="no"])
 CPPFLAGS="${save_CPPFLAGS}"
+if test "${ac_cv_my_have_sdl}" != "no"; then
+  AC_DEFINE(USE_SDL, 1, Define to 1 to use SDL_image)
+fi
 AM_CONDITIONAL(USE_SDL, test "${ac_cv_my_have_sdl}" = "yes")
 
 # Use OpenCV?
@@ -82,6 +85,9 @@ AC_CHECK_HEADERS(cv.h,
  [ac_cv_my_have_opencv="yes"],
  [ac_cv_my_have_opencv="no"])
 CPPFLAGS="${save_CPPFLAGS}"
+if test "${ac_cv_my_have_opencv}" != "no"; then
+  AC_DEFINE(USE_OPENCV, 1, Define to 1 to use OpenCV)
+fi
 AM_CONDITIONAL(USE_OPENCV, test "${ac_cv_my_have_opencv}" = "yes")
 
 # Use Imlib2?