feat: add system feature to use pre-installed CTranslate2 library

Introduced a new `system` feature to skip compiling CTranslate2 and instead link to an existing shared library. Updated build script, feature flags, and README with usage details.

Author: jkawamoto

README.md

@@ -83,6 +83,8 @@ If you want to use platform-specific default features, use the `ct2rs-platform`
 
 - `whisper`: Enables [Whisper](https://huggingface.co/docs/transformers/model_doc/whisper) model support
 - `hub`: Enables [HuggingFace Hub](https://huggingface.co/docs/hub) integration
+- `system`: Skip compiling CTranslate2 and use the system's pre-installed shared library instead (requires setting the
+  appropriate environment variables to locate the CTranslate2 shared library)
 
 ### Platform Specific Features
 

ct2rs/Cargo.toml

@@ -75,6 +75,7 @@ whisper = [
     "all-tokenizers",
 ]
 hub = ["dep:hf-hub"]
+system = ["intel-onemkl-prebuild?/system", "openblas-src?/system"]
 
 # Features to select backends.
 cuda = []

ct2rs/build.rs

@@ -31,15 +31,7 @@ enum Os {
     Unknown,
 }
 
-fn main() {
-    println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-changed=src/sys");
-    println!("cargo:rerun-if-changed=include");
-    println!("cargo:rerun-if-changed=CTranslate2");
-    add_search_paths("LIBRARY_PATH");
-    println!("cargo:rerun-if-env-changed=CMAKE_INCLUDE_PATH");
-    add_search_paths("CMAKE_LIBRARY_PATH");
-
+fn build_ctranslate2() {
     let mut cmake = Config::new("CTranslate2");
     match env::var("CMAKE_PARALLEL") {
         Ok(job_n) => {
@@ -198,6 +190,57 @@ fn main() {
 
     let ctranslate2 = cmake.build();
     link_libraries(ctranslate2.join("build"));
+}
+
+fn link_system_libraries() {
+    println!("cargo:rustc-link-lib=ctranslate2");
+    if cfg!(target_arch = "x86_64") {
+        println!("cargo:rustc-link-lib=cpu_features");
+    }
+    if cfg!(feature = "cuda") {
+        println!("cargo:rustc-link-lib=static=cudart_static");
+    }
+    if cfg!(feature = "cudnn") {
+        println!("cargo:rustc-link-lib=cudnn");
+    }
+    if !cfg!(feature = "cuda-dynamic-loading") {
+        if cfg!(target_os = "windows") {
+            println!("cargo:rustc-link-lib=static=cublas");
+            println!("cargo:rustc-link-lib=static=cublasLt");
+        }else{
+            println!("cargo:rustc-link-lib=static=cublas_static");
+            println!("cargo:rustc-link-lib=static=cublasLt_static");
+            println!("cargo:rustc-link-lib=static=culibos");
+        }
+    }
+    if cfg!(feature = "accelerate") {
+        println!("cargo:rustc-link-lib=framework=Accelerate");
+    }
+    if cfg!(feature = "dnnl") {
+        println!("cargo:rustc-link-lib=dnnl");
+    }
+    if cfg!(feature = "openmp-runtime-comp") {
+        println!("cargo:rustc-link-lib=gomp");
+    }
+    if cfg!(feature = "openmp-runtime-intel") {
+        println!("cargo:rustc-link-lib=iomp5");
+    }
+}
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=src/sys");
+    println!("cargo:rerun-if-changed=include");
+    println!("cargo:rerun-if-changed=CTranslate2");
+    add_search_paths("LIBRARY_PATH");
+    println!("cargo:rerun-if-env-changed=CMAKE_INCLUDE_PATH");
+    add_search_paths("CMAKE_LIBRARY_PATH");
+
+    if cfg!(feature = "system") {
+        link_system_libraries();
+    } else {
+        build_ctranslate2()
+    }
 
     cxx_build::bridges([
         "src/sys/types.rs",