Teach ParseFlags that -stdlib goes in CXXFLAGS.
https://github.com/SCons/scons/pull/4520
--- SCons/Environment.py.orig	2024-03-17 19:26:15.000000000 -0500
+++ SCons/Environment.py	2024-05-05 13:14:13.000000000 -0500
@@ -1022,6 +1022,8 @@
                     else:
                         key = 'CFLAGS'
                     mapping[key].append(arg)
+                elif arg[:8] == '-stdlib=':
+                    mapping['CXXFLAGS'].append(arg)
                 elif arg[0] == '+':
                     mapping['CCFLAGS'].append(arg)
                     mapping['LINKFLAGS'].append(arg)