Move the echo detection code to a macro in libtool.m4. The code

Move the echo detection code to a macro in libtool.m4.  The code
at the top ofltconfig.in is ugly, but will go away when the rest
of ltconfig is migrated.

* Makefile.am (libtool):  Pass the value of $ECHO to ltconfig as
an environment variable.
* libtool.m4 (_LT_AC_PROG_ECHO_BACKSLASH): Quote the ECHO value
correctly so that it inserts the correct code into configure.in
suitably quoted for M4 expansion.  This code needs to be near
the top of configure.in as it calls itself recursively, so I
am using AC_DIVERSION_NOTICE (or simply NOTICE on CVS
Autoconf).  The value of ECHO is also substitued into the
Makefile to be used when Make calls ltconfig to generate
libtool, so another layer of quotation is required for the
Makefile.
(AC_LIBTOOL_SETUP):  Use it.
* ltconfig.in: Accept a value for ECHO from the environment, and
carefully substitue for $0 if it is present in the ECHO value.
Because ECHO was quoted for Make, when ltconfig is called by
configure, it is overquoted, and one layer of quotes must be
stripped.