Restrict the undo mangling hack to gnu target
Restrict the undo mangling hack to gnu target

This is not needed at least for Clang 3.9 (which is required to parse
stdlib headers of MSVC 2015). This hack leads to incorrect mangling
result there.

catdesk/rust-bindgen,catdesk/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen,catdesk/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,jdub/rust-bindgen,catdesk/rust-bindgen,jdub/rust-bindgen