Index: llvm/trunk/utils/lit/CMakeLists.txt
===================================================================
--- llvm/trunk/utils/lit/CMakeLists.txt
+++ llvm/trunk/utils/lit/CMakeLists.txt
@@ -1,7 +1,7 @@
 # The configured file is not placed in the correct location
 # until the tests are run as we need to copy it into
 # a copy of the tests folder
-configure_file("tests/lit.site.cfg.in" "lit.site.cfg" @ONLY)
+configure_lit_site_cfg("tests/lit.site.cfg.in" "lit.site.cfg")
 
 # Lit's test suite creates output files next to the sources which makes the
 # source tree dirty. This is undesirable because we do out of source builds.
Index: llvm/trunk/utils/lit/tests/lit.site.cfg.in
===================================================================
--- llvm/trunk/utils/lit/tests/lit.site.cfg.in
+++ llvm/trunk/utils/lit/tests/lit.site.cfg.in
@@ -1,8 +1,19 @@
-## Autogenerated by LLVM/Clang configuration.
-# Do not edit!
+@LIT_SITE_CFG_IN_HEADER@
+
+import sys
+
 config.llvm_src_root = "@LLVM_SOURCE_DIR@"
 config.llvm_obj_root = "@LLVM_BINARY_DIR@"
-config.llvm_tools_dir = "@LLVM_TOOLS_BINARY_DIR@"
+config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
+
+# Support substitution of the tools_dir with user parameters.
+# This is used when we can't determine the tool dir at configuration time.
+try:
+    config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
+except KeyError:
+    e = sys.exc_info()[1]
+    key, = e.args
+    lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LLVM_BINARY_DIR@/utils/lit/tests/lit.cfg")