It's used at build time in 102 crates (of which 98 optionally, 3 directly). It's used only as a dev dependency in 4 crates.

Depender (with downloads and own rev deps) pnacl-build-helper version
4.4K 1 libressl-pnacl-sys build ^1.4
1 libvpx-sys build ^1.4
ppapi build *