scons: pass -isystem instead of -I for third party library includes

SConstruct

@@ -159,6 +159,8 @@ else:
     from cross_compile import *
     setup_cross_compile(env)
 
+env.Tool("system_include")
+
 if 'HOME' in os.environ:
     env['ENV']['HOME'] = os.environ['HOME']
 

scons/system_include.py

@@ -0,0 +1,20 @@
+def exists():
+    return True
+
+def make_incflags(paths, RDirs):
+    result = []
+    for path in paths:
+       if not str(path).startswith("#"):
+           for rdir in RDirs((path,)):
+               result.append("-isystem")
+               result.append(str(rdir))
+       else:
+           for rdir in RDirs((path,)):
+               result.append("-I" + str(rdir))
+    return result
+
+def generate(env):
+    if "gcc" in env["TOOLS"]:
+        env["make_incflags"] = make_incflags
+        env["INCPREFIX"] = ""
+        env["_CPPINCFLAGS"] = "$( ${_concat(INCPREFIX, CPPPATH, INCSUFFIX, __env__, lambda x : make_incflags(x, RDirs), TARGET, SOURCE)} $)"