Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -570,6 +570,10 @@
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static")
 endif()
 
+# Override the default target with an environment variable named by LLVM_TARGET_TRIPLE_ENV.
+set(LLVM_TARGET_TRIPLE_ENV CACHE STRING "The name of environment variable to override default target. Disabled by blank.")
+mark_as_advanced(LLVM_TARGET_TRIPLE_ENV)
+
 # All options referred to from HandleLLVMOptions have to be specified
 # BEFORE this include, otherwise options will not be correctly set on
 # first cmake run
Index: llvm/trunk/cmake/modules/AddLLVM.cmake
===================================================================
--- llvm/trunk/cmake/modules/AddLLVM.cmake
+++ llvm/trunk/cmake/modules/AddLLVM.cmake
@@ -1133,6 +1133,19 @@
 
   set(LIT_SITE_CFG_IN_HEADER  "## Autogenerated from ${input}\n## Do not edit!")
 
+  # Override config_target_triple (and the env)
+  if(LLVM_TARGET_TRIPLE_ENV)
+    # This is expanded into the heading.
+    string(CONCAT LIT_SITE_CFG_IN_HEADER "${LIT_SITE_CFG_IN_HEADER}\n\n"
+      "import os\n"
+      "target_env = \"${LLVM_TARGET_TRIPLE_ENV}\"\n"
+      "config.target_triple = config.environment[target_env] = os.environ.get(target_env, \"${TARGET_TRIPLE}\")\n"
+      )
+
+    # This is expanded to; config.target_triple = ""+config.target_triple+""
+    set(TARGET_TRIPLE "\"+config.target_triple+\"")
+  endif()
+
   configure_file(${input} ${output} @ONLY)
 endfunction()
 
Index: llvm/trunk/include/llvm/Config/config.h.cmake
===================================================================
--- llvm/trunk/include/llvm/Config/config.h.cmake
+++ llvm/trunk/include/llvm/Config/config.h.cmake
@@ -356,6 +356,9 @@
 /* Define if this is Win32ish platform */
 #cmakedefine LLVM_ON_WIN32 ${LLVM_ON_WIN32}
 
+/* Define if overriding target triple is enabled */
+#cmakedefine LLVM_TARGET_TRIPLE_ENV "${LLVM_TARGET_TRIPLE_ENV}"
+
 /* Define if we have the Intel JIT API runtime support library */
 #cmakedefine01 LLVM_USE_INTEL_JITEVENTS
 
Index: llvm/trunk/lib/Support/Unix/Host.inc
===================================================================
--- llvm/trunk/lib/Support/Unix/Host.inc
+++ llvm/trunk/lib/Support/Unix/Host.inc
@@ -45,5 +45,11 @@
     TargetTripleString += getOSVersion();
   }
 
+  // Override the default target with an environment variable named by LLVM_TARGET_TRIPLE_ENV.
+#if defined(LLVM_TARGET_TRIPLE_ENV)
+  if (const char *EnvTriple = std::getenv(LLVM_TARGET_TRIPLE_ENV))
+    TargetTripleString = EnvTriple;
+#endif
+
   return Triple::normalize(TargetTripleString);
 }
Index: llvm/trunk/lib/Support/Windows/Host.inc
===================================================================
--- llvm/trunk/lib/Support/Windows/Host.inc
+++ llvm/trunk/lib/Support/Windows/Host.inc
@@ -18,5 +18,13 @@
 using namespace llvm;
 
 std::string sys::getDefaultTargetTriple() {
-  return Triple::normalize(LLVM_DEFAULT_TARGET_TRIPLE);
+  const char *Triple = LLVM_DEFAULT_TARGET_TRIPLE;
+
+  // Override the default target with an environment variable named by LLVM_TARGET_TRIPLE_ENV.
+#if defined(LLVM_TARGET_TRIPLE_ENV)
+  if (const char *EnvTriple = std::getenv(LLVM_TARGET_TRIPLE_ENV))
+    Triple = EnvTriple;
+#endif
+
+  return Triple::normalize(Triple);
 }