$(ECHO) is not defined using libtool 2

[Peter]

After an upgrade to libtool 2, I found that libtool.m4 (used by AC_PROG_LIBTOOL or LT_INIT) $(ECHO) is no longer defined in Makefiles. Instead there is a $(lt_ECHO).

I think yat should work with both libtool 1 and 2. Either we should remove our use of $(ECHO) and fall back to a simple echo. Or we need to define $(ECHO) ourselves. Note that if we use a simple AC_SUBST([ECHO],[echo]), we should be sure that we don't block libtools definition. Preferably, of course, we should try to use the variable found by libtool.

[Peter]

(In [1761]) fixes #482