SCons: Fix BoolVariable import for SCons 4.8.0+

Author: akien-mga

tools/ios.py

@@ -2,7 +2,7 @@
 import sys
 import subprocess
 import common_compiler_flags
-from SCons.Variables import *
+from SCons.Variables import BoolVariable
 
 if sys.version_info < (3,):
 

tools/linux.py

@@ -1,5 +1,5 @@
 import common_compiler_flags
-from SCons.Variables import *
+from SCons.Variables import BoolVariable
 from SCons.Tool import clang, clangxx
 
 

tools/windows.py

@@ -2,7 +2,7 @@
 import my_spawn
 import common_compiler_flags
 from SCons.Tool import msvc, mingw
-from SCons.Variables import *
+from SCons.Variables import BoolVariable
 
 
 def options(opts):