Use Darwin libtool's -no_warning_for_no_symbols if available to silence the "has no symbols" link warning

[kubamracek]

Summary

Building compiler-rt on Darwin produces dozens of meaningless warnings about object files having no symbols during static archive creation. This is very intentional as compiler-rt uses #ifdefs to conditionally compile platform-specific code, and we even have a .cpp source file that only contains static asserts to make sure the environment is configured right. On Linux, this situation is fine and no warning is produces. This patch adds a libtool version detection and if it's new enough, we'll use the -no_warning_for_no_symbols flag that suppresses this warning. Build logs should be much cleaner now!

[kubamracek]

Note that cctools version 862 was shipped with Xcode 6.0.

[kubamracek]

ping

[beanz]

One minor nit below, otherwise LGTM.

CMakeLists.txt

77

You don't need to set the variable to empty, an unset variable will evaluate to empty anyways.

[kubamracek]

Thanks!

CMakeLists.txt

if(CMAKE_HOST_APPLE AND APPLE)
   if(NOT CMAKE_LIBTOOL OR NOT EXISTS CMAKE_LIBTOOL)
     find_program(CMAKE_LIBTOOL NAMES libtool)
   endif()
 
   get_property(languages GLOBAL PROPERTY ENABLED_LANGUAGES)
   if(CMAKE_LIBTOOL)
     set(CMAKE_LIBTOOL ${CMAKE_LIBTOOL} CACHE PATH "libtool executable")
     message(STATUS "Found libtool - ${CMAKE_LIBTOOL}")
+
+    set(LIBTOOL_NO_WARNING_FLAG "")
+    execute_process(COMMAND ${CMAKE_LIBTOOL} -V
+      OUTPUT_VARIABLE LIBTOOL_V_OUTPUT
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if("${LIBTOOL_V_OUTPUT}" MATCHES ".*cctools-([0-9.]+).*")
+      string(REGEX REPLACE ".*cctools-([0-9.]+).*" "\\1" LIBTOOL_VERSION
+        ${LIBTOOL_V_OUTPUT})
+      if(NOT LIBTOOL_VERSION VERSION_LESS "862")
+        set(LIBTOOL_NO_WARNING_FLAG "-no_warning_for_no_symbols")
+      endif()
+    endif()
+    
     foreach(lang ${languages})
       set(CMAKE_${lang}_CREATE_STATIC_LIBRARY
-        "${CMAKE_LIBTOOL} -static -o <TARGET> <LINK_FLAGS> <OBJECTS> ")
+        "${CMAKE_LIBTOOL} -static ${LIBTOOL_NO_WARNING_FLAG} -o <TARGET> \
+        <LINK_FLAGS> <OBJECTS> ")
     endforeach()
   endif()
 
   # If DYLD_LIBRARY_PATH is set we need to set it on archiver commands
   if(DYLD_LIBRARY_PATH)
     set(dyld_envar "DYLD_LIBRARY_PATH=${DYLD_LIBRARY_PATH}")
     foreach(lang ${languages})
       foreach(cmd ${CMAKE_${lang}_CREATE_STATIC_LIBRARY})