Use openmp-sys instead of -fopenmp flag

linker-args do not propagate transitively
(rust-lang/cargo#9554). Hence, -fopenmp would
need to be set in every crate that uses this crate.
The openmp-sys crate instead searches for the libraries and adds them
as link-lib, which is picked up by crates using this crate.

Cargo.toml

@@ -13,6 +13,7 @@ categories = ["api-bindings", "science"]
 
 [dependencies]
 num-complex = "0.4"
+openmp-sys = "1.2"
 
 [build-dependencies]
 cmake = "0.1"

build.rs

@@ -9,7 +9,6 @@ fn main() {
     // Link it
     println!("cargo:rustc-link-search={}", dst.join("lib").display());
     println!("cargo:rustc-link-lib=hptt");
-    println!("cargo:rustc-link-arg=-fopenmp");
     println!("cargo:rustc-link-lib=stdc++");
 
     // Generate bindings

src/lib.rs

@@ -1,3 +1,5 @@
+extern crate openmp_sys;
+
 mod hptt {
     include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
 }