added option to build conf file to choose arrayfire backend

Earlier, all the built backends were of ArrayFire submodule were
linked with rust wrapper. This created issues when multiple backends
were linked at the same time.

This commit, adds the option `use_backend` to `build.conf` which
dictates the backend ([cpu | cuda | opencl]) used by the rust
wrapper irrespective of arrayfire backends built by the build script.

Author: 9prady9

build.conf

@@ -1,11 +1,13 @@
 {
+    "use_backend": "cuda",
+
     "use_lib": false,
     "lib_dir": "/usr/local/lib",
     "inc_dir": "/usr/local/include",
 
     "build_type": "Release",
     "build_threads": "4",
-    "build_cuda": "OFF",
+    "build_cuda": "ON",
     "build_opencl": "ON",
     "build_cpu": "ON",
     "build_examples": "OFF",

build.rs

@@ -10,8 +10,13 @@ use std::path::PathBuf;
 use std::process::Command;
 use std::convert::AsRef;
 
+#[allow(dead_code)]
 #[derive(RustcDecodable)]
 struct Config {
+    // below variable dictates which
+    // backend library is used by rust wrapper
+    use_backend: String,
+
     // Use the existing lib if it exists
     use_lib: bool,
     lib_dir: String,
@@ -81,13 +86,15 @@ fn read_file(file_name: &std::path::PathBuf) -> String {
         .open(&file_path);
 
     let mut file = match options {
-            Ok(file) => file,
+            Ok(file)=> file,
             Err(..) => panic!("error reading file"),
         };
 
     let mut s = String::new();
-    file.read_to_string(&mut s);
-    return s.to_string()
+    match file.read_to_string(&mut s) {
+        Ok(_)   => s,
+        Err(..) => panic!("Error reading file to a string"),
+    }
 }
 
 fn read_conf(conf_file: &std::path::PathBuf) -> Config {
@@ -332,7 +339,9 @@ fn blob_backends(conf: &Config, build_dir: &std::path::PathBuf) -> (Vec<String>,
         backend_dirs.push(build_dir.join("package/lib").to_str().to_owned().unwrap().to_string());
     }
 
-    if conf.build_cuda == "ON" {
+    if conf.use_backend == "cpu" {
+        backends.push("afcpu".to_string());
+    } else if conf.use_backend == "cuda" {
         backends.push("afcuda".to_string());
         backends.push("nvvm".to_string());
         if cfg!(windows) {
@@ -342,13 +351,7 @@ fn blob_backends(conf: &Config, build_dir: &std::path::PathBuf) -> (Vec<String>,
             backend_dirs.push(format!("{}/lib64", conf.cuda_sdk));
             backend_dirs.push(format!("{}/nvvm/lib64", conf.cuda_sdk));
         }
-    }
-
-    if conf.build_cpu == "ON" {
-        backends.push("afcpu".to_string());
-    }
-
-    if conf.build_opencl == "ON" {
+    } else if conf.use_backend == "opencl" {
         backends.push(("afopencl".to_string()));
         backends.push("OpenCL".to_string());
         if cfg!(windows) {

src/lib.rs

@@ -10,7 +10,6 @@ use libc::c_longlong;
 use std::ops::Index;
 use std::ops::Add;
 
-#[link(name="afcpu")]
 extern {
     fn af_set_device(device: c_int) -> c_int;