Replace deprecated INCLUDES by AM_CPPFLAGS

tests/Makefile.am

@@ -37,5 +37,5 @@ unit_test_client_LDADD = $(common_ldflags)
 version_SOURCES = version.c
 version_LDADD = $(common_ldflags)
 
-INCLUDES = -I$(top_srcdir) -I$(top_srcdir)/src
+AM_CPPFLAGS = -I$(top_srcdir) -I$(top_srcdir)/src
 CLEANFILES = *~