scons: Use the current python executable for code generation.

Less susceptible to be broken.

src/gallium/auxiliary/SConscript

@@ -34,14 +34,14 @@ env.CodeGenerate(
     target = 'util/u_format_table.c',
     script = '#src/gallium/auxiliary/util/u_format_table.py',
     source = ['#src/gallium/auxiliary/util/u_format.csv'],
-    command = 'python $SCRIPT $SOURCE > $TARGET'
+    command = python_cmd + ' $SCRIPT $SOURCE > $TARGET'
 )
 
 env.CodeGenerate(
     target = 'util/u_half.c',
     script = 'util/u_half.py',
     source = [],
-    command = 'python $SCRIPT > $TARGET'
+    command = python_cmd + ' $SCRIPT > $TARGET'
 )
 
 env.Depends('util/u_format_table.c', [

src/gallium/drivers/llvmpipe/SConscript

@@ -1,3 +1,4 @@
+from sys import executable as python_cmd
 import distutils.version
 
 Import('*')
@@ -16,7 +17,7 @@ env.CodeGenerate(
 	target = 'lp_tile_soa.c',
 	script = 'lp_tile_soa.py',
 	source = ['#src/gallium/auxiliary/util/u_format.csv'],
-	command = 'python $SCRIPT $SOURCE > $TARGET'
+	command = python_cmd + ' $SCRIPT $SOURCE > $TARGET'
 )
 
 # XXX: Our dependency scanner only finds depended modules in relative dirs.