README: Document python2 dependency

Much of the tools and configure makefile actually have a python2
dependency; specify this.  It also assumes that `python` points to `python2`;
document how to work around this on systems where this is false.

Also update second version requirement listed to match the first.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

README

@@ -46,7 +46,7 @@ provided by your OS distributor:
         - GCC 4.8 or later
         - GNU Binutils 2.24 or later
     * Development install of zlib (e.g., zlib-dev)
-    * Development install of Python v2.4 or later (e.g., python-dev)
+    * Development install of Python 2, v2.4 or later (e.g., python-dev)
     * Development install of curses (e.g., libncurses-dev)
     * Development install of openssl (e.g., openssl-dev)
     * Development install of x11 (e.g. xorg-x11-dev)
@@ -177,10 +177,16 @@ Python Runtime Libraries
 
 Various tools, such as pygrub, have the following runtime dependencies:
 
-    * Python 2.3 or later.
+    * Python 2, v2.4 or later.
           URL:    http://www.python.org/
           Debian: python
 
+Note that the build system expects `python` to be python2.  If your system
+has `python` pointing to python3 (as in the case of Arch Linux or Anaconda),
+you'll need to specify a path to a python2 binary when running configure:
+
+    PYTHON=/usr/bin/python2 ./configure
+
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