Haiku: rework build.rs logic for curl-sys (#375)

Due to several issues, the Haiku build of rustc and cargo is cross-compiled
from a Linux host. In this official build, it is prefered to link to the
libcurl build that is being distributed as an official Haiku port. This
change adds the logic to link to the official libcurl build when
cross-compiling. The previous logic was flawed, in the sense that it would
rely on the hosts curl-config to determine whether http2 support was available.

When doing a native build, pkg_config can be used to determine whether
libcurl is available on the platform, thus removing the need for separate logic
when building curl-sys on Haiku itself.

Author: nielx

curl-sys/build.rs

@@ -9,6 +9,7 @@ use std::path::{Path, PathBuf};
 use std::process::Command;
 
 fn main() {
+    let host = env::var("HOST").unwrap();
     let target = env::var("TARGET").unwrap();
     let windows = target.contains("windows");
 
@@ -19,13 +20,18 @@ fn main() {
         return println!("cargo:rustc-flags=-l curl");
     }
 
+    // When cross-compiling for Haiku, use the system's default supplied
+    // libcurl (it supports http2). This is in the case where rustc and
+    // cargo are built for Haiku, which is done from a Linux host.
+    if host != target && target.contains("haiku") {
+        return println!("cargo:rustc-flags=-l curl");
+    }
+
     // If the static-curl feature is disabled, probe for a system-wide libcurl.
     if !cfg!(feature = "static-curl") {
-        // OSX and Haiku ships libcurl by default, so we just use that version
+        // OSX ships libcurl by default, so we just use that version
         // so long as it has the right features enabled.
-        if (target.contains("apple") || target.contains("haiku"))
-            && (!cfg!(feature = "http2") || curl_config_reports_http2())
-        {
+        if target.contains("apple") && (!cfg!(feature = "http2") || curl_config_reports_http2()) {
             return println!("cargo:rustc-flags=-l curl");
         }