[CMake] [Darwin] Use libtool instead of ar && ranlib

[beanz]

Summary

Using libtool instead of ar and ranlib on Darwin shaves a minute off my clang build. This is because on Darwin libtool is optimized to give hints to the kernel about filesystem interactions that allow it to be faster.

llvm/trunk/CMakeLists.txt

set(PACKAGE_VERSION
     "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX}")
 endif()
 
 project(LLVM
   ${cmake_3_0_PROJ_VERSION}
   ${cmake_3_0_LANGUAGES}
   C CXX ASM)
 
+if(APPLE)
+  if(NOT CMAKE_LIBTOOL)
+    find_program(CMAKE_LIBTOOL NAMES libtool)
+  endif()
+  if(CMAKE_LIBTOOL)
+    set(CMAKE_LIBTOOL ${CMAKE_LIBTOOL} CACHE PATH "libtool executable")
+    message(STATUS "Found libtool - ${CMAKE_LIBTOOL}")
+    get_property(languages GLOBAL PROPERTY ENABLED_LANGUAGES)
+    foreach(lang ${languages})
+      set(CMAKE_${lang}_CREATE_STATIC_LIBRARY
+        "${CMAKE_LIBTOOL} -static -o <TARGET> <LINK_FLAGS> <OBJECTS> ")
+    endforeach()
+  endif()
+endif()
+
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs.")
 if(LLVM_PARALLEL_COMPILE_JOBS)
   if(CMAKE_VERSION VERSION_LESS 3.0 OR NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
     message(WARNING "Job pooling is only available with Ninja generators and CMake 3.0 and later.")
   else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${LLVM_PARALLEL_COMPILE_JOBS})