Include headers

[evetion]

Would it possible to distribute header files? Downstream builds using this library require headers on compiling with curl support. In JuliaGeo/GDALBuilder#12 we had to switch to building curl instead of referencing this build.

In practice this would mean make install instead of make install-exec? This would undo #10, apparently also generating docs. We could manually remove docs (rm -rf share/man) if needed.

If you agree, I can make a PR.

[visr]

Linking also to https://github.com/JuliaPackaging/Yggdrasil/blob/master/L/LibCURL/build_tarballs.jl for julia 1.3.