C API to the PineAPPL library

To use pineappl via the C API, you first need cargo-c to generate the C header. First run cargo install cargo-c to install it, and then install the C API:

cargo cinstall --release --prefix=${prefix}

Make sure to replace ${prefix} with the directory you want it installed to. This crate installs a header file, a library, and a pkg-config file, so make sure to set the necessary environment variables.

On Linux, you need to set at least PKG_CONFIG_PATH to the directory where the pineappl_capi.pc file is. It usually is in ${prefix}/lib/pkgconfig. If you've set it to right value the following command

pkg-config pineappl_capi --libs

should print the library flags needed to link against the library. If there's no output, double-check your installation and environment variables. Finally, it's probably necessary to set LD_LIBRARY_PATH to the directory where the PineAPPL shared/static library was installed to.