Rollup merge of rust-lang#39950 - ishitatsuyuki:stop-trashing-ldpath, r=alexcrichton

bootstrap: stop trashing (DY)LD_LIBRARY_PATH

This generated an ugly error with fakeroot before.

Author: eddyb

src/bootstrap/bootstrap.py

@@ -355,8 +355,12 @@ 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()):