scons: Move scons and python version checking into site_init.py.

In an effort to shrink and modularize the main scons files, this
change pulls the scons and python version checking code out of the
main file and into site_init.py which runs before the SConstruct
starts. This will be a place to put really generic code which has to
do with the very fundemental aspects of getting scons to work. Other
checks, like checks for particular tools or particular versions of
tools, will happen in other more specialized files.

Change-Id: Icd00ecadbe1141aef4dbadcf42d6ddef1f3a701f
Reviewed-on: https://gem5-review.googlesource.com/5561
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

SConstruct

@@ -78,35 +78,6 @@
 #
 ###################################################
 
-# Check for recent-enough Python and SCons versions.
-try:
-    # Really old versions of scons only take two options for the
-    # function, so check once without the revision and once with the
-    # revision, the first instance will fail for stuff other than
-    # 0.98, and the second will fail for 0.98.0
-    EnsureSConsVersion(0, 98)
-    EnsureSConsVersion(0, 98, 1)
-except SystemExit, e:
-    print """
-For more details, see:
-    http://gem5.org/Dependencies
-"""
-    raise
-
-# pybind11 requires python 2.7
-try:
-    EnsurePythonVersion(2, 7)
-except SystemExit, e:
-    print """
-You can use a non-default installation of the Python interpreter by
-rearranging your PATH so that scons finds the non-default 'python' and
-'python-config' first.
-
-For more details, see:
-    http://gem5.org/wiki/index.php/Using_a_non-default_Python_installation
-"""
-    raise
-
 # Global Python includes
 import itertools
 import os