clang_4: fix build w/gcc9

As noted in the incorporated patch, name collisions between lambda
arguments and lambda captures cause compilation failures in GCC9[1].

The issue has been fixed in a later version of CLang, unfortunately the
relevant patches weren't backported to the versions 3.9 and 4.0[2].

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2211
[2] https://github.com/llvm-mirror/clang/commit/bb177a9307360a211ca0111b49b160698395074c#diff-d63bede6ed85300ad2aaf4eaee571456L6278