• Add comments to note a clang bug and and the patched revision.
• Add a knob WITH_SYSTEM_CLANG to force build with clang from base system.

This is very useful when the user has patched the system clang, i.e.,
contrib/llvm/tools/clang/lib/Sema/SemaDeclCXX.cpp.

• Temporarily disable -Wshadow and -Wunused-parameter. These options caused

too many warnings for system boost and made it really hard to debug problems.

• Remove temporary clang++ options, which are no longer necessary.
• Remove a duplicate patch for solenv/gbuild/platform/com_GCC_defs.mk.