prefer versioned python binaries to non-versioned ones

On many systems, there are several parallel python
installations. There is usually a "python" symlink that
points to the currently active python version. That might
not be the one we need. Instead of picking up that one, we
now search for python$VERSION and as a last resort use
python.

Also, make sure found python lib/binaries match. This
fixes BZDNG-388.

cmake_support/FindPython.cmake

@@ -42,7 +42,7 @@ endmacro()
 
 
 macro(_find_python PYTHON_ROOT VERSION)
-  string(REPLACE "." "" _VERSION_NO_DOTS ${VERSION})
+  string(REPLACE "." "" _VERSION_NO_DOTS "${VERSION}")
   if(PYTHON_ROOT)
     find_library(PYTHON_LIBRARIES
       NAMES "python${_VERSION_NO_DOTS}" "python${VERSION}"
@@ -69,17 +69,17 @@ macro(_find_python PYTHON_ROOT VERSION)
 endmacro()
 
 macro(_find_python_bin PYTHON_ROOT VERSION)
-  string(REPLACE "." "" _VERSION_NO_DOTS ${VERSION})
+  string(REPLACE "." "" _VERSION_NO_DOTS "${VERSION}")
   if(PYTHON_ROOT)
     find_program(PYTHON_BINARY
-      NAMES "python" "python${_VERSION_NO_DOTS}" "python${VERSION}"
+      NAMES "python${_VERSION_NO_DOTS}" "python${VERSION}"
       HINTS "${PYTHON_ROOT}"
       PATH_SUFFIXES bin
       NO_SYSTEM_ENVIRONMENT_PATH NO_DEFAULT_PATH
     )
   else()
     find_program(PYTHON_BINARY
-      NAMES "python" "python${_VERSION_NO_DOTS}" "python${VERSION}"
+      NAMES "python${_VERSION_NO_DOTS}" "python${VERSION}"
       HINTS "${CMAKE_PREFIX_PATH}"
       PATH_SUFFIXES bin
     )  
@@ -105,6 +105,10 @@ macro(check_for_python_lib)
       endif()
     endforeach()
   endif()
+  # fallback to non-versioned naming scheme
+  if (NOT $PYTHON_LIBRARIES)
+    _find_python("${PYTHON_ROOT}" "")
+  endif()
 endmacro()
 
 macro(check_for_python_binary)
@@ -114,13 +118,19 @@ macro(check_for_python_binary)
     foreach(_VERSION ${PYTHON_VERSIONS})
       if(${PYTHON_MIN_VERSION} VERSION_LESS ${_VERSION})
         _find_python_bin("${PYTHON_ROOT}" "${_VERSION}")
-        if(PYTHON_LIBRARIES)
+        if(PYTHON_BINARY)
           set(PYTHON_VERSION "${_VERSION}")
+          # disallow all versions except for the one we just found. This makes
+          # sure we don't mismatch the python binary and the libraries.
+          set(PYTHON_VERSIONS "${_VERSION}")
           break()
         endif()
       endif()
     endforeach()
   endif()
+  if (NOT PYTHON_BINARY)
+    _find_python("${PYTHON_ROOT}" "")
+  endif()
 endmacro()
 
 if(NOT PYTHON_ROOT)
@@ -134,11 +144,13 @@ if(APPLE AND NOT PYTHON_IGNORE_FRAMEWORKS)
   check_for_python_framework()
 endif()
 
+# first check for python binary.
+check_for_python_binary()
+
 if(NOT PYTHON_FRAMEWORK_FOUND)
   check_for_python_lib()
 endif()
 
-check_for_python_binary()
 mark_as_advanced(
   PYTHON_LIBRARIES
   PYTHON_INCLUDE_PATH
@@ -159,6 +171,8 @@ if(PYTHON_LIBRARIES)
 endif()
 
 if (PYTHON_BINARY)
+  set(PYTHON_VERSION "${PYTHON_VERSION}"
+      CACHE STRING "Python Version" FORCE)
   set(PYTHON_BINARY "${PYTHON_BINARY}"
       CACHE FILEPATH "Python Binary" FORCE)
 endif()