safer check for sys.executable in sitecustomize.py

make sure it doesn't actually point to something else

since sitecustomize.py is put in a location found by pypy,
it breaks pypy by setting sys.executable to a path that is definitely wrong,
and may not even exist.

Closes Homebrew/homebrew#24581.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/requirements/python_dependency.rb

@@ -296,10 +296,10 @@ class PythonDependency < Requirement
       # This file is created by Homebrew and is executed on each python startup.
       # Don't print from here, or else python command line scripts may fail!
       # <https://github.com/mxcl/homebrew/wiki/Homebrew-and-Python>
+      import os
       import sys
 
       if sys.version_info[0] != #{version.major}:
-          import os
           # This can only happen if the user has set the PYTHONPATH for 3.x and run Python 2.x or vice versa.
           # Every Python looks at the PYTHONPATH variable and we can't fix it here in sitecustomize.py,
           # because the PYTHONPATH is evaluated after the sitecustomize.py. Many modules (e.g. PyQt4) are
@@ -310,7 +310,8 @@ class PythonDependency < Requirement
                '     You should `unset PYTHONPATH` to fix this.')
       else:
           # Only do this for a brewed python:
-          if sys.executable.startswith('#{HOMEBREW_PREFIX}'):
+          opt_executable = '#{HOMEBREW_PREFIX}/opt/#{python}/bin/#{xy}'
+          if os.path.realpath(sys.executable) == os.path.realpath(opt_executable):
               # Remove /System site-packages, and the Cellar site-packages
               # which we moved to lib/pythonX.Y/site-packages. Further, remove
               # HOMEBREW_PREFIX/lib/python because we later addsitedir(...).
@@ -331,7 +332,7 @@ class PythonDependency < Requirement
                   pass  # remember: don't print here. Better to fail silently.
 
               # Set the sys.executable to use the opt_prefix
-              sys.executable = '#{HOMEBREW_PREFIX}/opt/#{python}/bin/#{xy}'
+              sys.executable = opt_executable
 
           # Tell about homebrew's site-packages location.
           # This is needed for Python to parse *.pth.