[CMake] Support passing arguments to build tool for external projects

[andrewng]

Summary

Add CMake variable LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS to allow
arguments to be passed to the native tool used in CMake --build
invocations for external projects.

Can be used to pass extra arguments for enhanced versions of build
tools, e.g. distributed build options.

[MaskRay]

(I don't know much about CMake :/)

[andrewng]

Ping.

[beanz]

One minor nit, otherwise looks good.

llvm/CMakeLists.txt

179

We should probably mark this as an advanced option since I don't expect most users will use it.

[andrewng]

Address review comments.

[phosek]

LGTM

[beanz]

LGTM

[andrewng]

llvm/CMakeLists.txt

179

Thanks for the suggestion and I've also now learnt about mark_as_advanced.

llvm/CMakeLists.txt

if(CCACHE_PROGRAM)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
   else()
     message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
   endif()
 endif()
 
+set(LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS "" CACHE STRING
+  "Optional arguments for the native tool used in CMake --build invocations for external projects.")
+mark_as_advanced(LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS)
+
 option(LLVM_DEPENDENCY_DEBUGGING "Dependency debugging mode to verify correctly expressed library dependencies (Darwin only)" OFF)
 
 # Some features of the LLVM build may be disallowed when dependency debugging is
 # enabled. In particular you cannot use ccache because we want to force compile
 # operations to always happen.
 if(LLVM_DEPENDENCY_DEBUGGING)
   if(NOT CMAKE_HOST_APPLE)
     message(FATAL_ERROR "Dependency debugging is only currently supported on Darwin hosts.")

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

 include(ExternalProject)
 
 # llvm_ExternalProject_BuildCmd(out_var target)
 #   Utility function for constructing command lines for external project targets
 function(llvm_ExternalProject_BuildCmd out_var target bin_dir)
   cmake_parse_arguments(ARG "" "CONFIGURATION" "" ${ARGN})
   if(NOT ARG_CONFIGURATION)
     set(ARG_CONFIGURATION "$<CONFIG>")
   endif()
   if (CMAKE_GENERATOR MATCHES "Make")
     # Use special command for Makefiles to support parallelism.
     set(${out_var} "$(MAKE)" "-C" "${bin_dir}" "${target}" PARENT_SCOPE)
   else()
+    set(tool_args "${LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS}")
+    if(NOT tool_args STREQUAL "")
+      string(CONFIGURE "${tool_args}" tool_args @ONLY)
+      string(PREPEND tool_args "-- ")
+      separate_arguments(tool_args UNIX_COMMAND "${tool_args}")
+    endif()
     set(${out_var} ${CMAKE_COMMAND} --build ${bin_dir} --target ${target}
-                                    --config ${ARG_CONFIGURATION} PARENT_SCOPE)
+                                    --config ${ARG_CONFIGURATION} ${tool_args} PARENT_SCOPE)
   endif()
 endfunction()
 
 # is_msvc_triple(out_var triple)
 #   Checks whether the passed triple refers to an MSVC environment
 function(is_msvc_triple out_var triple)
   if (triple MATCHES ".*-windows-msvc.*")
     set(${out_var} TRUE PARENT_SCOPE)