Don't assume LIT_EXECUTABLE points to a Python script

Before this patch, the CMake build assumed LIT_EXECUTABLE pointed
to a Python script, not an executable.  If you were to pass in an
executable, such as the result of py2exe on lit.py, the build would
fall over.

With this patch, the CMake build assumes LIT_EXECUTABLE is an
executable.  You can continue setting it to lit.py, but it will
now use its shebang to find a Python interpreter.

Differential Revision: http://reviews.llvm.org/D7315

llvm-svn: 227994