libctf: require a Tcl capable of try/catch to run tests

libctf: require a Tcl capable of try/catch to run tests

The run_native_host_cmd implementation in testsuite/lib/ctf-lib.exp
uses try/catch, which are new in Tcl 8.6.  Require a Tcl that knows
that try exists, as suggested by Jan Beulich.

libctf/ChangeLog
2021-02-03  Nick Alcock  <nick.alcock@oracle.com>

	* configure.ac (EXPECT): Check for, in order to define...
	(TCL_TRY): ... this, if Tcl supports try/catch.
	* Makefile.am (TCL_TRY): Run the testsuite only if set.
	* configure: Regenerated.
	* Makefile.in: Likewise.