add extra checks for older clang on osx, closes #1660

scripts/functions/build_config

@@ -5,7 +5,7 @@ __rvm_setup_compile_environment()
   typeset __type
   typeset -a __types
   __types=(
-    setup requirements osx_gcc architectures gcc47 bison smartos sunos openbsd
+    setup requirements osx_gcc osx_clang architectures gcc47 bison smartos sunos openbsd
     auto_conf_make flags_docs flags_tcltk flags_shared_static flags_threads
   )
   for __type in "${__types[@]}"
@@ -110,6 +110,47 @@ __rvm_setup_compile_environment_osx_gcc()
   fi
 }
 
+__rvm_setup_compile_environment_osx_clang_installed() [[ -x /usr/bin/clang ]]
+
+__rvm_setup_compile_environment_osx_clang_latest()
+{
+  typeset __clang_version
+  __clang_version="$( /usr/bin/clang --version | grep -oE "clang-\d+\.\d+\.\d+" )" || return $?
+  __clang_version="${__clang_version#clang-}"
+  __rvm_version_compare "${__clang_version:-0}" -ge "425.0.24" || return $?
+  true # for osx
+}
+
+__rvm_setup_compile_environment_osx_clang()
+{
+  [[ "Darwin" == "$(uname)" ]] || return 0
+
+  if
+    [[ "$( __rvm_found_compiler )" =~ "clang" ]]
+  then
+    if
+      __rvm_version_compare "${_system_version}" -ge 10.7
+    then
+      __rvm_setup_compile_environment_osx_clang_installed ||
+      rvm_requiremnts_fail_or_run_action \
+        "Can not find 'Command Line Tools for Xcode',
+You should install the latest version from: http://connect.apple.com" \
+        true || return $?
+
+      __rvm_setup_compile_environment_osx_clang_latest ||
+      rvm_requiremnts_fail_or_run_action \
+        "Found older 'Command Line Tools for Xcode',
+You should install the latest version from: http://connect.apple.com" \
+        true || return $?
+    else
+      rvm_requiremnts_fail_or_run_action \
+        "You are using 'clang' which is known to be buggy on older OSX (like: ${_system_version}).
+Make sure 'gcc-4.2' is available and try again." \
+        true || return $?
+    fi
+  fi
+}
+
 __rvm_setup_compile_environment_bison()
 {
   (( ${rvm_autolibs_flag_number} > 1 )) || return 0