handle rhs leading slashes

toolchain/internal/configure.bzl

@@ -50,6 +50,12 @@ def llvm_register_toolchains():
     pass
 """)
 
+def _join(path1, path2):
+    if path1:
+        return paths.join(path1, path2.lstrip("/"))
+    else:
+        return path2
+
 def llvm_config_impl(rctx):
     _check_os_arch_keys(rctx.attr.sysroot)
     _check_os_arch_keys(rctx.attr.cxx_builtin_include_directories)
@@ -322,14 +328,14 @@ def _cc_toolchain_str(
         sysroot_prefix = "%sysroot%"
     if target_os == "linux":
         cxx_builtin_include_directories.extend([
-            paths.join(sysroot_prefix, "include"),
-            paths.join(sysroot_prefix, "usr/include"),
-            paths.join(sysroot_prefix, "usr/local/include"),
+            _join(sysroot_prefix, "/include"),
+            _join(sysroot_prefix, "/usr/include"),
+            _join(sysroot_prefix, "/usr/local/include"),
         ])
     elif target_os == "darwin":
         cxx_builtin_include_directories.extend([
-            paths.join(sysroot_prefix, "usr/include"),
-            paths.join(sysroot_prefix, "System/Library/Frameworks"),
+            _join(sysroot_prefix, "/usr/include"),
+            _join(sysroot_prefix, "/System/Library/Frameworks"),
         ])
     else:
         fail("Unreachable")