Handles libffi on the CMake build.

Patch by arrowdodger!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123976 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ExecutionEngine/Interpreter/CMakeLists.txt

@@ -1,5 +1,21 @@
+# If the user required a custom path for ffi headers, use it on its
+# abolute form (see config-ix.cmake):
+if( FFI_INCLUDE_DIR )
+  include_directories( ${FFI_INCLUDE_PATH} )
+endif()
+
+# If the user required a custom path for libffi, use it on its abolute
+# form (see config-ix.cmake):
+if( FFI_LIBRARY_DIR )
+  link_directories( ${FFI_LIBRARY_PATH} )
+endif()
+
 add_llvm_library(LLVMInterpreter
   Execution.cpp
   ExternalFunctions.cpp
   Interpreter.cpp
   )
+
+if( LLVM_ENABLE_FFI )
+  target_link_libraries( LLVMInterpreter ffi )
+endif()