[libcxx] Don't assume LIT_EXECUTABLE points to a Python script

[garious]

Summary

lit.py's shebang allows it to masquerade as an executable. The
CMake build ignores that and assumes it is a Python script. If
LIT_EXECUTABLE was actually an executable, the build would fall
over.

This patch cleans things up. It assumes LIT_EXECUTABLE is
an actual executable. If it is a Python script, we'll use its
shebang to find a Python interpreter.

[mclow.lists]

This looks fine to me.

[danalbert]

LGTM.

libcxx/trunk/test/CMakeLists.txt

 macro(pythonize_bool var)
   if (${var})
     set(${var} True)
   else()
     set(${var} False)
   endif()
 endmacro()
 
-include(FindPythonInterp)
-if(PYTHONINTERP_FOUND)
-  if(LIBCXX_BUILT_STANDALONE)
-    set(LIT_EXECUTABLE "" CACHE FILEPATH "Path to LLVM's lit.py.")
-    # Make sure we can use the console pool for recent cmake and ninja > 1.5
-    if(CMAKE_VERSION VERSION_LESS 3.1.20141117)
-      set(cmake_3_2_USES_TERMINAL)
-    else()
-      set(cmake_3_2_USES_TERMINAL USES_TERMINAL)
-    endif()
-  else()
-    set(LIT_EXECUTABLE "${CMAKE_SOURCE_DIR}/utils/lit/lit.py")
-  endif()
+set(LIT_EXECUTABLE "" CACHE FILEPATH "Path to LLVM's llvm-lit.")
+
+  include(FindPythonInterp)
+  if(PYTHONINTERP_FOUND)
+    set(LIT_EXECUTABLE
+        "${PYTHON_EXECUTABLE} ${CMAKE_SOURCE_DIR}/utils/lit/lit.py")
+  else()
+    message(WARNING "Could not find Python, cannot set LIT_EXECUTABLE.")
+  endif()
 
+if (LIT_EXECUTABLE)
   set(LIT_ARGS_DEFAULT "-sv --show-unsupported --show-xfail")
   if (MSVC OR XCODE)
     set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
   endif()
   set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}"
       CACHE STRING "Default options for lit")
   set(LIT_ARGS "${LLVM_LIT_ARGS}")
   separate_arguments(LIT_ARGS)
if (LIT_EXECUTABLE)
   set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
 
   configure_file(
     ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
     ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
     @ONLY)
 
   add_custom_target(check-libcxx
-    COMMAND ${PYTHON_EXECUTABLE}
-            ${LIT_EXECUTABLE}
+    COMMAND ${LIT_EXECUTABLE}
             ${LIT_ARGS}
             ${CMAKE_CURRENT_BINARY_DIR}
     DEPENDS cxx
     COMMENT "Running libcxx tests"
     ${cmake_3_2_USES_TERMINAL})
 else()
-  message(WARNING "Could not find Python, no check target will be available!")
+  message(WARNING
+          "LIT_EXECUTABLE not set, no check-libcxx target will be available!")
 endif()