Re-add support for running individual tests when built with LLVM3.3

It seems that the LLVM configure script no longer
looks for tclsh which was used to execute individual tests. E.g.

$ cd test
$ make TESTONE=Runtime/POSIX/DirConsistency.c check-one VERBOSE=1

This prevented the above from working. This commit fixes this by
having our configure script look for tclsh instead. The path_tclsh.m4
macro is taken from the projects/sample/autoconf/m4/ in LLVM3.3