swift234

[buildbot-script] make it possible to force the CMake executable and install prefix

If no SWIFT_CMAKE is specified, fall back to the one in PATH, then
/usr/local/bin/cmake.  Presumably these last two are the same on the
buildbot, but if not, we can always set
SWIFT_CMAKE=/usr/local/bin/cmake in the buildbot's environment

Swift SVN r10056