[CMake][libcxx] Move Python check to main CMake file

[phosek]

Summary

This is to make sure this check is called even when building as part of LLVM runtimes when we are doing standalone but not out of tree build.

[beanz]

LGTM!

CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(PACKAGE_VERSION 4.0.0svn)
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "[e-mail]")
 
   # Find the LLVM sources and simulate LLVM CMake options.
   include(HandleOutOfTreeLLVM)
 endif()
 
+if (LIBCXX_STANDALONE_BUILD)
+  include(FindPythonInterp)
+  if( NOT PYTHONINTERP_FOUND )
+    message(WARNING "Failed to find python interpreter. "
+                    "The libc++ test suite will be disabled.")
+    set(LLVM_INCLUDE_TESTS OFF)
+  endif()
+endif()
+
 # Require out of source build.
 include(MacroEnsureOutOfSourceBuild)
 MACRO_ENSURE_OUT_OF_SOURCE_BUILD(
  "${PROJECT_NAME} requires an out of source build. Please create a separate
  build directory and run 'cmake /path/to/${PROJECT_NAME} [options]' there."
  )
 
 #===============================================================================

cmake/Modules/HandleOutOfTreeLLVM.cmake

else()
       set(LLVM_ON_WIN32 0)
     endif()
   endif()
   if (LLVM_FOUND)
     include(AddLLVM OPTIONAL)
   endif()
 
   # LLVM Options --------------------------------------------------------------
-  include(FindPythonInterp)
-  if( NOT PYTHONINTERP_FOUND )
-    message(WARNING "Failed to find python interpreter. "
-                    "The libc++ test suite will be disabled.")
-    set(LLVM_INCLUDE_TESTS OFF)
-  endif()
-
   if (NOT DEFINED LLVM_INCLUDE_TESTS)
     set(LLVM_INCLUDE_TESTS ${LLVM_FOUND})
   endif()
   if (NOT DEFINED LLVM_INCLUDE_DOCS)
     set(LLVM_INCLUDE_DOCS ${LLVM_FOUND})
   endif()
   if (NOT DEFINED LLVM_ENABLE_SPHINX)
     set(LLVM_ENABLE_SPHINX OFF)