Add func_append_quoted and do inline func_append substitutions.

Add func_append_quoted and do inline func_append substitutions.

* libtool/config/ltmain.m4sh: Replace all occurrences of
`foo="$foo bar"' with func_append. No decorator comment was
added since /func_append [a-zA-Z0-9_]* "/ is already an
excellent match regexp, and additional comments would only
complicate things.
(func_append_quoted): New function that quotes a value with
func_quote_for_eval before appending it.  To avoid quoting the
whitespace in the passed value, a single whitespace is added
automatically.  Changed callers where possible.
* libtool/m4/libtool.m4 (_LT_PROG_XSI_SHELLFNS): Perform a
`+=' implementation substitution for func_append_quoted.  More
importantly, systematically replace matching func_append calls
with inline `+=' when supported (for maximum speed), and
longhand `foo="$foo bar"' otherwise to avoid the function
overhead.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>