root6: either make it work on 10.6 (or declare it broken)

[mojca (Mojca Miklavec)]

I'm isolating this issue from #43917.

The build log is here:

• ​https://build.macports.org/builders/buildports-snowleopard-x86_64/builds/27095

-- The C compiler identification is Clang 3.4.0
-- The CXX compiler identification is Clang 3.4.0
-- Check for working C compiler: /opt/local/bin/clang-mp-3.4
-- Check for working C compiler: /opt/local/bin/clang-mp-3.4 -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /opt/local/bin/clang++-mp-3.4
-- Check for working CXX compiler: /opt/local/bin/clang++-mp-3.4 -- broken
CMake Error at /opt/local/share/cmake-2.8/Modules/CMakeTestCXXCompiler.cmake:54 (message):
  The C++ compiler "/opt/local/bin/clang++-mp-3.4" is not able to compile a
  simple test program.

  It fails with the following output:

   Change Dir: /opt/local/var/macports/build/_opt_mports_dports_science_root6/root6/work/build/CMakeFiles/CMakeTmp

  

  Run Build Command:/usr/bin/make "cmTryCompileExec828241131/fast"

  /usr/bin/make -f CMakeFiles/cmTryCompileExec828241131.dir/build.make
  CMakeFiles/cmTryCompileExec828241131.dir/build

  /opt/local/bin/cmake -E cmake_progress_report
  /opt/local/var/macports/build/_opt_mports_dports_science_root6/root6/work/build/CMakeFiles/CMakeTmp/CMakeFiles
  1

  Building CXX object
  CMakeFiles/cmTryCompileExec828241131.dir/testCXXCompiler.cxx.o

  /opt/local/bin/clang++-mp-3.4 -pipe -Os -arch x86_64 -stdlib=libc++
  -isysroot /Developer/SDKs/MacOSX10.6.sdk -mmacosx-version-min=10.6 -o
  CMakeFiles/cmTryCompileExec828241131.dir/testCXXCompiler.cxx.o -c
  /opt/local/var/macports/build/_opt_mports_dports_science_root6/root6/work/build/CMakeFiles/CMakeTmp/testCXXCompiler.cxx


  Linking CXX executable cmTryCompileExec828241131

  /opt/local/bin/cmake -E cmake_link_script
  CMakeFiles/cmTryCompileExec828241131.dir/link.txt --verbose=1

  /opt/local/bin/clang++-mp-3.4 -pipe -Os -arch x86_64 -stdlib=libc++
  -isysroot /Developer/SDKs/MacOSX10.6.sdk -mmacosx-version-min=10.6
  -Wl,-search_paths_first -Wl,-headerpad_max_install_names -L/opt/local/lib
  -Wl,-headerpad_max_install_names -arch x86_64
  CMakeFiles/cmTryCompileExec828241131.dir/testCXXCompiler.cxx.o -o
  cmTryCompileExec828241131

  ld: library not found for -lc++

  clang: error: linker command failed with exit code 1 (use -v to see
  invocation)

  make[1]: *** [cmTryCompileExec828241131] Error 1

  make: *** [cmTryCompileExec828241131/fast] Error 2

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:5 (project)


-- Configuring incomplete, errors occurred!

The problem seems to be

ld: library not found for -lc++