findtimezone.c needs to know this setting too. Per Peter Eisentraut.
override CPPFLAGS := -DFRONTEND -I$(libpq_srcdir) -I$(top_srcdir)/src/timezone $(CPPFLAGS)
+# use system timezone data?
+ifneq (,$(with_system_tzdata))
+override CPPFLAGS += '-DSYSTEMTZDIR="$(with_system_tzdata)"'
+endif
+
OBJS= initdb.o findtimezone.o localtime.o encnames.o pqsignal.o $(WIN32RES)
all: initdb