Create the regtest reference files only when necessary

This avoid recreating the ref files every time an individual test
is run from the command line.

Originally committed as revision 24113 to svn://svn.ffmpeg.org/ffmpeg/trunk

Makefile

@@ -289,6 +289,9 @@ $(LAVF_TESTS) $(LAVFI_TESTS): regtest-ref
 REFFILE = $(SRC_PATH)/tests/ref/$(1)/$(2:regtest-%=%)
 RESFILE = tests/data/$(2:regtest-%=%).$(1).regression
 
+AREF = tests/data/acodec.ref.wav
+VREF = tests/data/vsynth1.ref.yuv tests/data/vsynth2.ref.yuv
+
 define VCODECTEST
 	@echo "TEST VCODEC $(1:regtest-%=%)"
 	$(SRC_PATH)/tests/codec-regression.sh $(1) vsynth1 tests/vsynth1 "$(TARGET_EXEC)" "$(TARGET_PATH)"
@@ -301,11 +304,13 @@ define ACODECTEST
 endef
 
 regtest-ref: regtest-aref regtest-vref
+regtest-aref: $(AREF)
+regtest-vref: $(VREF)
 
-regtest-vref: ffmpeg$(EXESUF) tests/vsynth1/00.pgm tests/vsynth2/00.pgm
+$(VREF): ffmpeg$(EXESUF) tests/vsynth1/00.pgm tests/vsynth2/00.pgm
 	@$(call VCODECTEST,vref)
 
-regtest-aref: ffmpeg$(EXESUF) tests/data/asynth1.sw
+$(AREF): ffmpeg$(EXESUF) tests/data/asynth1.sw
 	@$(call ACODECTEST,aref)
 
 $(VCODEC_TESTS): tests/tiny_psnr$(HOSTEXESUF)