FindPythonInterp: fix version parsing

On dashmacmini2 the test showed output like this:

-- Found PythonInterp: /usr/bin/python (found version "Unknown option: --
usage: /usr/bin/python [option] ... [-c cmd | file | -] [arg] ...
Try `python -h' for more information.")

On my machine where python outputs "Python 2.7" this worked, but
PYTHON_VERSION_MAJOR, PYTHON_VERSION_MINOR, and PYTHON_VERSION_PATCH were all
set to "2.7".

Add some checks that the version output has the expected form before using it.

Modules/FindPythonInterp.cmake

@@ -89,11 +89,15 @@ endif()
 # determine python version string
 if(PYTHON_EXECUTABLE)
     execute_process(COMMAND "${PYTHON_EXECUTABLE}" --version ERROR_VARIABLE _VERSION OUTPUT_QUIET ERROR_STRIP_TRAILING_WHITESPACE)
-    string(REPLACE "Python " "" PYTHON_VERSION_STRING "${_VERSION}")
-    string(REGEX REPLACE "^([0-9]+)\\.[0-9]+\\.[0-9]+.*" "\\1" PYTHON_VERSION_MAJOR "${PYTHON_VERSION_STRING}")
-    string(REGEX REPLACE "^[0-9]+\\.([0-9])+\\.[0-9]+.*" "\\1" PYTHON_VERSION_MINOR "${PYTHON_VERSION_STRING}")
-    string(REGEX REPLACE "^[0-9]+\\.[0-9]+\\.([0-9]+).*" "\\1" PYTHON_VERSION_PATCH "${PYTHON_VERSION_STRING}")
-endif()
+    if(_VERSION MATCHES "^Python [0-9]+\\.[0-9]+.*")
+        string(REPLACE "Python " "" PYTHON_VERSION_STRING "${_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}")
+        endif()
+    endif()
+endif(PYTHON_EXECUTABLE)
 
 # handle the QUIETLY and REQUIRED arguments and set PYTHONINTERP_FOUND to TRUE if
 # all listed variables are TRUE