FindPythonInterp: rework the version detection

There are versions out there that neither understand --version nor -V. Try a
completely different approach: execute a small python script that prints the
version number (and only that) in an easily reusable way using
sys.version_info. This is documented to work since Python 2.0. Use sys.version
for older versions, which is documented to exist since 1.5. If even that
doesn't work then simply assume we are on 1.4.0.

Modules/FindPythonInterp.cmake

@@ -15,6 +15,7 @@
 #=============================================================================
 # Copyright 2005-2010 Kitware, Inc.
 # Copyright 2011 Bjoern Ricks <[email protected]>
+# Copyright 2012 Rolf Eike Beer <[email protected]>
 #
 # Distributed under the OSI-approved BSD License (the "License");
 # see accompanying file Copyright.txt for details.
@@ -88,24 +89,42 @@ endif()
 
 # determine python version string
 if(PYTHON_EXECUTABLE)
-    execute_process(COMMAND "${PYTHON_EXECUTABLE}" --version
-                    ERROR_VARIABLE _VERSION
+    execute_process(COMMAND "${PYTHON_EXECUTABLE}" -c
+                            "import sys; sys.stdout.write(';'.join([str(x) for x in sys.version_info[:3]]))"
+                    OUTPUT_VARIABLE _VERSION
                     RESULT_VARIABLE _PYTHON_VERSION_RESULT
-                    OUTPUT_QUIET
-                    ERROR_STRIP_TRAILING_WHITESPACE)
-    if(_PYTHON_VERSION_RESULT)
-        execute_process(COMMAND "${PYTHON_EXECUTABLE}" -V
-                        ERROR_VARIABLE _VERSION
+                    ERROR_QUIET)
+    if(NOT _PYTHON_VERSION_RESULT)
+        string(REPLACE ";" "." PYTHON_VERSION_STRING "${_VERSION}")
+        list(GET _VERSION 0 PYTHON_VERSION_MAJOR)
+        list(GET _VERSION 1 PYTHON_VERSION_MINOR)
+        list(GET _VERSION 2 PYTHON_VERSION_PATCH)
+        if(PYTHON_VERSION_PATCH EQUAL 0)
+            # it's called "Python 2.7", not "2.7.0"
+            string(REGEX REPLACE "\\.0$" "" PYTHON_VERSION_STRING "${PYTHON_VERSION_STRING}")
+        endif()
+    else()
+        # sys.version predates sys.version_info, so use that
+        execute_process(COMMAND "${PYTHON_EXECUTABLE}" -c "import sys; sys.stdout.write(sys.version)"
+                        OUTPUT_VARIABLE _VERSION
                         RESULT_VARIABLE _PYTHON_VERSION_RESULT
-                        OUTPUT_QUIET
-                        ERROR_STRIP_TRAILING_WHITESPACE)
-    endif(_PYTHON_VERSION_RESULT)
-    if(NOT _PYTHON_VERSION_RESULT AND _VERSION MATCHES "^Python [0-9]+\\.[0-9]+.*")
-        string(REPLACE "Python " "" PYTHON_VERSION_STRING "${_VERSION}")
-        string(REGEX REPLACE "^([0-9]+)\\.[0-9]+.*" "\\1" PYTHON_VERSION_MAJOR "${PYTHON_VERSION_STRING}")
-        string(REGEX REPLACE "^[0-9]+\\.([0-9])+.*" "\\1" PYTHON_VERSION_MINOR "${PYTHON_VERSION_STRING}")
-        if(PYTHON_VERSION_STRING MATCHES "^[0-9]+\\.[0-9]+\\.[0-9]+.*")
-            string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" PYTHON_VERSION_PATCH "${PYTHON_VERSION_STRING}")
+                        ERROR_QUIET)
+        if(NOT _PYTHON_VERSION_RESULT)
+            string(REGEX REPLACE " .*" "" PYTHON_VERSION_STRING "${_VERSION}")
+            string(REGEX REPLACE "^([0-9]+)\\.[0-9]+.*" "\\1" PYTHON_VERSION_MAJOR "${PYTHON_VERSION_STRING}")
+            string(REGEX REPLACE "^[0-9]+\\.([0-9])+.*" "\\1" PYTHON_VERSION_MINOR "${PYTHON_VERSION_STRING}")
+            if(PYTHON_VERSION_STRING MATCHES "^[0-9]+\\.[0-9]+\\.[0-9]+.*")
+                string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" PYTHON_VERSION_PATCH "${PYTHON_VERSION_STRING}")
+            else()
+                set(PYTHON_VERSION_PATCH "0")
+            endif()
+        else()
+            # sys.version was first documented for Python 1.5, so assume
+            # this is older.
+            set(PYTHON_VERSION_STRING "1.4")
+            set(PYTHON_VERSION_MAJOR "1")
+            set(PYTHON_VERSION_MAJOR "4")
+            set(PYTHON_VERSION_MAJOR "0")
         endif()
     endif()
     unset(_PYTHON_VERSION_RESULT)