avutil/Makefile: Apply CFLAGS for compilation

Fixes "make tools/crypto_bench.o".

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/Makefile

@@ -262,6 +262,6 @@ TESTPROGS-$(HAVE_LZO1X_999_COMPRESS) += lzo
 TOOLS = crypto_bench ffhash ffeval ffescape
 
 tools/crypto_bench$(EXESUF): ELIBS += $(if $(VERSUS),$(subst +, -l,+$(VERSUS)),)
-tools/crypto_bench$(EXESUF): CFLAGS += -DUSE_EXT_LIBS=0$(if $(VERSUS),$(subst +,+USE_,+$(VERSUS)),)
+tools/crypto_bench.o: CFLAGS += -DUSE_EXT_LIBS=0$(if $(VERSUS),$(subst +,+USE_,+$(VERSUS)),)
 
 $(SUBDIR)tests/lzo$(EXESUF): ELIBS = -llzo2