Use a script for creating the clang++ executable.

The previous method used the DESTDIR environment variable at configure
time, but sometimes it is only available at install time. See PR8397.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@116689 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/driver/CMakeLists.txt b/tools/driver/CMakeLists.txt
index ec6e9c6..b5c7b14 100644
--- a/tools/driver/CMakeLists.txt
+++ b/tools/driver/CMakeLists.txt
@@ -35,7 +35,6 @@
 
 if(UNIX)
   set(CLANGXX_LINK_OR_COPY create_symlink)
-  set(CLANGXX_DESTDIR $ENV{DESTDIR}/)
 else()
   set(CLANGXX_LINK_OR_COPY copy)
 endif()
@@ -51,4 +50,4 @@
   RUNTIME DESTINATION bin)
 
 # Create the clang++ symlink at installation time.
-install(CODE "execute_process(COMMAND \"${CMAKE_COMMAND}\" -E ${CLANGXX_LINK_OR_COPY} \"${CMAKE_INSTALL_PREFIX}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}\" \"${CLANGXX_DESTDIR}${CMAKE_INSTALL_PREFIX}/bin/clang++${CMAKE_EXECUTABLE_SUFFIX}\")")
+install(SCRIPT clang_symlink.cmake -DCMAKE_INSTALL_PREFIX=\"${CMAKE_INSTALL_PREFIX}\")
diff --git a/tools/driver/clang_symlink.cmake b/tools/driver/clang_symlink.cmake
new file mode 100644
index 0000000..d3f11d3
--- /dev/null
+++ b/tools/driver/clang_symlink.cmake
@@ -0,0 +1,18 @@
+# We need to execute this script at installation time because the
+# DESTDIR environment variable may be unset at configuration time.
+# See PR8397.
+
+if(UNIX)
+  set(CLANGXX_LINK_OR_COPY create_symlink)
+  set(CLANGXX_DESTDIR $ENV{DESTDIR})
+else()
+  set(CLANGXX_LINK_OR_COPY copy)
+endif()
+
+set(bindir "${CLANGXX_DESTDIR}${CMAKE_INSTALL_PREFIX}/bin/")
+set(clang "${bindir}clang${CMAKE_EXECUTABLE_SUFFIX}")
+set(clangxx "${bindir}clang++${CMAKE_EXECUTABLE_SUFFIX}")
+
+message("Creating clang++ executable based on ${clang}")
+
+execute_process(COMMAND "${CMAKE_COMMAND}" -E ${CLANGXX_LINK_OR_COPY} "${clang}" "${clangxx}")