py2-dxr: misc fixes for Darwin support

re2 on Darwin/OSX only supports Clang + libc++. libstdc++ and libc++
are not ABI compatible, causing build issues.
```
libc++    : operator<<(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, re2::StringPiece const&)
libstdc++ : operator<<(std::ostream&, re2::StringPiece const&)
```
Modify re2 `libre2.symbols.darwin` symbols file to be compatible with
libstdc++.

`libclang-index-plugin.so` Makefile seems to be out-of-date. Removed
unsupported and not needed linker flags. To resolve all undefined
symbols we need to link to LLVM, Clang and zlib.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>