build: use bin path override if no python is found in PATH

On systems with no "python" in the PATH, e.g. FreeBSD, we should always
create a python symlink in get_bin_override(). Otherwise, configure
fails with the following error:

Traceback (most recent call last):
  File "./configure", line 1461, in <module>
    bin_override = get_bin_override()
  File "./configure", line 1360, in get_bin_override
    if os.path.realpath(which('python')) == os.path.realpath(sys.executable):
  File "/usr/local/lib/python2.7/posixpath.py", line 375, in realpath
    path, ok = _joinrealpath('', filename, {})
  File "/usr/local/lib/python2.7/posixpath.py", line 381, in _joinrealpath
    if isabs(rest):
  File "/usr/local/lib/python2.7/posixpath.py", line 54, in isabs
    return s.startswith('/')
AttributeError: 'NoneType' object has no attribute 'startswith'

configure

@@ -1357,7 +1357,9 @@ def get_bin_override():
   # sys.executable. This directory will be prefixed to the PATH, so that
   # other tools that shell out to `python` will use the appropriate python
 
-  if os.path.realpath(which('python')) == os.path.realpath(sys.executable):
+  which_python = which('python')
+  if (which_python and 
+      os.path.realpath(which_python) == os.path.realpath(sys.executable)):
     return
 
   bin_override = os.path.abspath('out/tools/bin')