Disable ThinLTO for dist builds.

Dist builds should always be as fast as we can make them, and since
those run on CI we don't care quite as much for the build being somewhat
slower. As such, we don't automatically enable ThinLTO on builds for the
dist builders.

src/bootstrap/configure.py

@@ -70,6 +70,7 @@ o("emscripten", None, "compile the emscripten backend as well as LLVM")
 # Optimization and debugging options. These may be overridden by the release
 # channel, etc.
 o("optimize", "rust.optimize", "build optimized rust code")
+o("thinlto", "rust.thinlto", "build Rust with ThinLTO enabled")
 o("optimize-llvm", "llvm.optimize", "build optimized LLVM")
 o("llvm-assertions", "llvm.assertions", "build LLVM with assertions")
 o("debug-assertions", "rust.debug-assertions", "build with debugging assertions")