Make the use of gold linker configurable

Currently, ld.gold is always used for linking if found on the
system. There are some cases however when one may want to opt out
from using it. This patch adds the boolean field `rustc-with-gold`
to the `[tools]` section of `.servobuild`, which if set false,
disables the use of ld.gold.

python/servo/command_base.py

@@ -101,6 +101,7 @@ def resolverelative(category, key):
         if not self.config["tools"]["system-cargo"]:
             self.config["tools"]["cargo-root"] = path.join(
                 context.sharedir, "cargo", self.cargo_build_id())
+        self.config["tools"].setdefault("rustc-with-gold", True)
 
         self.config.setdefault("build", {})
         self.config["build"].setdefault("android", False)
@@ -308,8 +309,9 @@ def build_env(self, gonk=False, hosts_file_path=None):
 
         env['RUSTDOC'] = path.join(self.context.topdir, 'etc', 'rustdoc-with-private')
 
-        if subprocess.call(['which', 'ld.gold'], stdout=PIPE, stderr=PIPE) == 0:
-            env['RUSTC'] = path.join(self.context.topdir, 'etc', 'rustc-with-gold')
+        if self.config["tools"]["rustc-with-gold"]:
+            if subprocess.call(['which', 'ld.gold'], stdout=PIPE, stderr=PIPE) == 0:
+                env['RUSTC'] = path.join(self.context.topdir, 'etc', 'rustc-with-gold')
 
         return env
 

servobuild.example

@@ -23,6 +23,10 @@ system-rust = false
 system-cargo = false
 #cargo-root = "/path/to/cargo"
 
+# If rustc-with-gold is true, will try to find and use gold linker with rustc.
+# Defaults to true
+rustc-with-gold = true
+
 [build]
 # Set "mode = dev" or use `mach build --dev` to build the project with warning.
 # or Set "mode = release" or use `mach build --release` for optimized build.