add environment variable to force usage of pkg-config (#175)

* add environment variable to force usage of pkg-config

* add pkg-config feature

* don't print rerun cargo instruction with non-cargo feature

Author: figsoda

zstd-safe/zstd-sys/Cargo.toml

@@ -54,7 +54,6 @@ default-features = false
 features = ["runtime", "which-rustfmt"]
 
 [build-dependencies.pkg-config]
-optional = true
 version = "0.3"
 
 [build-dependencies.cc]
@@ -71,6 +70,7 @@ debug = [] # Enable zstd debug logs
 experimental = [] # Expose experimental ZSTD API
 legacy = [] # Enable legacy ZSTD support (for versions < zstd-0.8)
 non-cargo = [] # Silence cargo-specific build flags
+pkg-config = []
 std = [] # Use std types instead of libc in bindgen
 zstdmt = [] # Enable multi-thread support (with pthread)
 thin = [] # Optimize binary by size

zstd-safe/zstd-sys/build.rs

@@ -38,7 +38,6 @@ fn generate_bindings(defs: Vec<&str>, headerpaths: Vec<PathBuf>) {
 #[cfg(not(feature = "bindgen"))]
 fn generate_bindings(_: Vec<&str>, _: Vec<PathBuf>) {}
 
-#[cfg(feature = "pkg-config")]
 fn pkg_config() -> (Vec<&'static str>, Vec<PathBuf>) {
     let library = pkg_config::Config::new()
         .statik(true)
@@ -48,11 +47,6 @@ fn pkg_config() -> (Vec<&'static str>, Vec<PathBuf>) {
     (vec!["PKG_CONFIG"], library.include_paths)
 }
 
-#[cfg(not(feature = "pkg-config"))]
-fn pkg_config() -> (Vec<&'static str>, Vec<PathBuf>) {
-    unimplemented!()
-}
-
 #[cfg(not(feature = "legacy"))]
 fn set_legacy(_config: &mut cc::Build) {}
 
@@ -217,6 +211,9 @@ fn compile_zstd() {
 }
 
 fn main() {
+    #[cfg(not(feature = "non-cargo"))]
+    println!("cargo:rerun-if-env-changed=ZSTD_SYS_USE_PKG_CONFIG");
+
     let target_arch =
         std::env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
     let target_os = std::env::var("CARGO_CFG_TARGET_OS").unwrap_or_default();
@@ -226,7 +223,9 @@ fn main() {
     }
 
     // println!("cargo:rustc-link-lib=zstd");
-    let (defs, headerpaths) = if cfg!(feature = "pkg-config") {
+    let (defs, headerpaths) = if cfg!(feature = "pkg-config")
+        || env::var_os("ZSTD_SYS_USE_PKG_CONFIG").is_some()
+    {
         pkg_config()
     } else {
         if !Path::new("zstd/lib").exists() {