diff --git a/src/bootstrap/bootstrap.py b/src/bootstrap/bootstrap.py
index 7ca7ef4bd720c..58de5706e94f6 100644
--- a/src/bootstrap/bootstrap.py
+++ b/src/bootstrap/bootstrap.py
@@ -342,8 +342,12 @@ def build_bootstrap(self):
         env = os.environ.copy()
         env["CARGO_TARGET_DIR"] = build_dir
         env["RUSTC"] = self.rustc()
-        env["LD_LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib")
-        env["DYLD_LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib")
+        env["LD_LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib") + \
+                                 (os.pathsep + env["LD_LIBRARY_PATH"]) \
+                                 if "LD_LIBRARY_PATH" in env else ""
+        env["DYLD_LIBRARY_PATH"] = os.path.join(self.bin_root(), "lib") + \
+                                   (os.pathsep + env["DYLD_LIBRARY_PATH"]) \
+                                   if "DYLD_LIBRARY_PATH" in env else ""
         env["PATH"] = os.path.join(self.bin_root(), "bin") + \
                       os.pathsep + env["PATH"]
         if not os.path.isfile(self.cargo()):