Set CXX for CUDA, if conditions are met.

Thanks to Dev for the suggestion.

We've had a long-standing "issue" with Archlinux that, because it distributes
really new compilers, these may not play nicely with what nvcc expects. To get
around that, there's a gcc and g++ in $CUDA_PATH/bin. I've manually gotten
around compilation issues by setting $CXX to this, but now $CXX will be set if:

1) $CXX is not already set
2) $CUDA_PATH is set
3) $CUDA_PATH/bin/g++ exists

build.rs

@@ -168,6 +168,19 @@ mod gpu {
                 .cudart("shared") // We handle linking cudart statically
                 .include("src/fee/gpu/")
                 .file("src/fee/gpu/fee.cu");
+            // If $CXX is not set but $CUDA_PATH is, search for
+            // $CUDA_PATH/bin/g++ and if it exists, set that as $CXX.
+            if !env::var_os("CXX").is_some() {
+                // Unlike above, we care about $CUDA_PATH being unicode.
+                if let Ok(cuda_path) = env::var("CUDA_PATH") {
+                    // Look for the g++ that CUDA wants.
+                    let compiler = std::path::PathBuf::from(cuda_path).join("bin/g++");
+                    if compiler.exists() {
+                        println!("cargo:warning=Setting $CXX to {}", compiler.display());
+                        env::set_var("CXX", compiler.into_os_string());
+                    }
+                }
+            }
             // Loop over each arch and sm
             for arch in arches {
                 for &sm in &sms {