build: use "python" instead of "python3" as interpreter name

This works regardless whether "python" starts a Python 2 or Python 3
interpreter.

TOOLS/file2string.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 
 # Convert the contents of a file into a C string constant.
 # Note that the compiler will implicitly add an extra 0 byte at the end

TOOLS/matroska.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 """
 Generate C definitions for parsing Matroska files.
 Can also be used to directly parse Matroska files and display their contents.

TOOLS/vdpau_functions.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env python
 
 # Generate vdpau_template.c
 

configure

@@ -1019,8 +1019,8 @@ echocheck "working compiler"
 cflag_check "" || die "Compiler is not functioning correctly. Check your installation and custom CFLAGS $CFLAGS ."
 echo "yes"
 
-echocheck "python3"
-command_check python3 -c '' || die "Python 3 is not functioning correctly. Check your installation and PATH."
+echocheck "python"
+command_check python -c '' || die "Python is not functioning correctly. Check your installation and PATH."
 echo yes
 
 if test -z "$_target" && x86 ; then