Better way of conditioning the sanitizer builds

Previously the build would take the presence of the LLVM_CONFIG envvar to
mean that the sanitizers should be built, but this is a common envvar that
could be set for reasons unrelated to the rustc sanitizers.

This commit adds a new envvar RUSTC_BUILD_SANITIZERS and uses it instead.

src/bootstrap/compile.rs

@@ -212,6 +212,7 @@ pub fn std_cargo(builder: &Builder<'_>,
                 emscripten: false,
             });
             cargo.env("LLVM_CONFIG", llvm_config);
+            cargo.env("RUSTC_BUILD_SANITIZERS", "1");
         }
 
         cargo.arg("--features").arg(features)

src/librustc_asan/build.rs

@@ -4,6 +4,9 @@ use build_helper::sanitizer_lib_boilerplate;
 use cmake::Config;
 
 fn main() {
+    if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
+        return;
+    }
     if let Some(llvm_config) = env::var_os("LLVM_CONFIG") {
         build_helper::restore_library_path();
 

src/librustc_lsan/build.rs

@@ -4,6 +4,9 @@ use build_helper::sanitizer_lib_boilerplate;
 use cmake::Config;
 
 fn main() {
+    if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
+        return;
+    }
     if let Some(llvm_config) = env::var_os("LLVM_CONFIG") {
         build_helper::restore_library_path();
 

src/librustc_msan/build.rs

@@ -4,6 +4,9 @@ use build_helper::sanitizer_lib_boilerplate;
 use cmake::Config;
 
 fn main() {
+    if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
+        return;
+    }
     if let Some(llvm_config) = env::var_os("LLVM_CONFIG") {
         build_helper::restore_library_path();
 

src/librustc_tsan/build.rs

@@ -4,6 +4,9 @@ use build_helper::sanitizer_lib_boilerplate;
 use cmake::Config;
 
 fn main() {
+    if env::var("RUSTC_BUILD_SANITIZERS") != Ok("1".to_string()) {
+        return;
+    }
     if let Some(llvm_config) = env::var_os("LLVM_CONFIG") {
         build_helper::restore_library_path();