build: add --with-python option

This adds a new --with-python option to allow configuration of the python binary
for building.  If not specified, $PATH will be searched for common python binary
names (python, python2, python3).  If specified, and the path is not executable,
it will attempt to search $PATH.

Signed-off-by: Saleem Abdulrasool <compnerd@compnerd.org>
Reviewed-by: Eric Christopher &lt;echristo@gmail.com&gt;, Daniel Dunbar <daniel@zuster.org>
llvm-svn: 173890