Use the system liblz4 if found via pkg-config

Compare to:

https://github.com/alexcrichton/bzip2-rs/pull/58/files

lz4-sys/Cargo.toml

@@ -13,3 +13,4 @@ libc = "0.2"
 
 [build-dependencies]
 cc = "1.1"
+pkg-config = "0.3.9"

lz4-sys/build.rs

@@ -15,6 +15,17 @@ fn main() {
 }
 
 fn run() -> Result<(), Box<dyn Error>> {
+    let target = get_from_env("TARGET")?;
+
+    if !target.contains("windows")
+        && pkg_config::Config::new()
+            .cargo_metadata(true)
+            .probe("liblz4")
+            .is_ok()
+    {
+        return Ok(());
+    }
+
     let mut compiler = cc::Build::new();
     compiler
         .file("liblz4/lib/lz4.c")
@@ -24,7 +35,6 @@ fn run() -> Result<(), Box<dyn Error>> {
         // We always compile the C with optimization, because otherwise it is 20x slower.
         .opt_level(3);
 
-    let target = get_from_env("TARGET")?;
     if target.contains("windows") {
         if target == "i686-pc-windows-gnu" {
             // Disable auto-vectorization for 32-bit MinGW target.