allow system-defined CPPFLAGS in /tests

diff --git a/tests/Makefile b/tests/Makefile
index d11b7fe..25f00b8 100644 (file)
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -39,7 +39,7 @@ CFLAGS  += -Wall -Wextra -Wundef -Wcast-qual -Wcast-align -Wshadow \
            -Wswitch-enum -Wdeclaration-after-statement -Wstrict-prototypes \
            -Wpointer-arith -Wstrict-aliasing=1
 CFLAGS  += $(DEBUGFLAGS) $(MOREFLAGS)
-CPPFLAGS:= -I$(LZ4DIR) -I$(PRGDIR) -DXXH_NAMESPACE=LZ4_
+CPPFLAGS+= -I$(LZ4DIR) -I$(PRGDIR) -DXXH_NAMESPACE=LZ4_
 FLAGS    = $(CFLAGS) $(CPPFLAGS) $(LDFLAGS)