8081447: System JPEG builds include in-tree jpeglib.h, resulting in build failure

The include directive should be conditional on the use of the system JPEG library.

Reviewed-by: prr, ihse

jdk/make/lib/Awt2dLibraries.gmk

@@ -491,7 +491,7 @@ $(eval $(call SetupNativeCompilation,BUILD_LIBJAVAJPEG, \
     SRC := $(LIBJAVAJPEG_SRC), \
     INCLUDE_FILES := $(BUILD_LIBJAVAJPEG_INCLUDE_FILES), \
     OPTIMIZATION := HIGHEST, \
-    CFLAGS := $(CFLAGS_JDKLIB) $(addprefix -I, $(LIBJAVAJPEG_SRC)) \
+    CFLAGS := $(CFLAGS_JDKLIB) $(BUILD_LIBJAVAJPEG_HEADERS) \
         $(LIBJAVA_HEADER_FLAGS) \
         -I$(SUPPORT_OUTPUTDIR)/headers/java.desktop, \
     DISABLED_WARNINGS_gcc := clobbered parentheses array-bounds, \