sdl: Fix include files install.

Patch sdl-config rpath setting rather than setting exec-prefix and
using DESTDIR as the SDL makefile uses prefix for some files,
and exec-prefix for others leading to double $(STAGING_DIR) prefix.

package/sdl/sdl.mk

@@ -56,7 +56,6 @@ $(SDL_DIR)/.configured: $(SDL_DIR)/.unpacked
 		--host=$(GNU_TARGET_NAME) \
 		--build=$(GNU_HOST_NAME) \
 		--prefix=$(STAGING_DIR)/usr \
-		--exec-prefix=/usr \
 		--sysconfdir=/etc \
 		--localstatedir=/var \
 		--enable-pulseaudio=no \
@@ -83,9 +82,9 @@ $(SDL_DIR)/.compiled: $(SDL_DIR)/.configured $(SDL_DIRECTFB_TARGET)
 	touch $@
 
 $(STAGING_DIR)/usr/lib/libSDL.so: $(SDL_DIR)/.compiled
-	$(MAKE) DESTDIR=$(STAGING_DIR) -C $(SDL_DIR) install
-# sdl-config uses -Lexec_prefix/lib instead of -Lprefix/lib - fix it
-	$(SED) 's^-L\$${exec_prefix}^-L\$${prefix}^' \
+	$(MAKE) -C $(SDL_DIR) install
+# use correct rpath for linking
+	$(SED) 's^libdir=\$${exec_prefix}^libdir=/usr^' \
 		$(STAGING_DIR)/usr/bin/sdl-config
 	touch -c $@