Tweak cmake config-ix to work when LLVM is embedded in another cmake source tree.

[v.g.vassilev]

Summary

We build LLVM as part of our project. It resides in a subfolder and thus using CMAKE_SOURCE_DIR doesn't work for us. It points to the top level cmake folder.

Patch by Bertrand Bellenot.

I am adding Chris Bieneman as a reviewer since I saw him making cmake related commits. Please let me know if I need to add somebody else.

[beanz]

LGTM.

[v.g.vassilev]

Landed in r272481.

cmake/config-ix.cmake

   set(LLVM_NATIVE_ASMPRINTER LLVMInitialize${LLVM_NATIVE_ARCH}AsmPrinter)
 
   # We don't have an ASM parser for all architectures yet.
-  if (EXISTS ${CMAKE_SOURCE_DIR}/lib/Target/${LLVM_NATIVE_ARCH}/AsmParser/CMakeLists.txt)
+  if (EXISTS ${PROJECT_SOURCE_DIR}/lib/Target/${LLVM_NATIVE_ARCH}/AsmParser/CMakeLists.txt)
     set(LLVM_NATIVE_ASMPARSER LLVMInitialize${LLVM_NATIVE_ARCH}AsmParser)
   endif ()
 
   # We don't have an disassembler for all architectures yet.
-  if (EXISTS ${CMAKE_SOURCE_DIR}/lib/Target/${LLVM_NATIVE_ARCH}/Disassembler/CMakeLists.txt)
+  if (EXISTS ${PROJECT_SOURCE_DIR}/lib/Target/${LLVM_NATIVE_ARCH}/Disassembler/CMakeLists.txt)
     set(LLVM_NATIVE_DISASSEMBLER LLVMInitialize${LLVM_NATIVE_ARCH}Disassembler)
   endif ()
 endif ()

   if(GO_EXECUTABLE STREQUAL "GO_EXECUTABLE-NOTFOUND")
     message(STATUS "Go bindings disabled.")
   else()
-    execute_process(COMMAND ${GO_EXECUTABLE} run ${CMAKE_SOURCE_DIR}/bindings/go/conftest.go
+    execute_process(COMMAND ${GO_EXECUTABLE} run ${PROJECT_SOURCE_DIR}/bindings/go/conftest.go
                     RESULT_VARIABLE GO_CONFTEST)
     if(GO_CONFTEST STREQUAL "0")
       set(LLVM_BINDINGS "${LLVM_BINDINGS} go")