[compiler-rt] Make cxxabi availability configurable in cmake

[fjricci]

Summary

Allow for use of cxxabi to be disabled from cmake.

This will make sanitizers usable when c++ abi is unavailable.

CMakeLists.txt

 endif()
 
 option(COMPILER_RT_DEBUG "Build runtimes with full debug info" OFF)
 option(COMPILER_RT_EXTERNALIZE_DEBUGINFO
   "Generate dSYM files and strip executables and libraries (Darwin Only)" OFF)
 # COMPILER_RT_DEBUG_PYBOOL is used by lit.common.configured.in.
 pythonize_bool(COMPILER_RT_DEBUG)
 
+if(APPLE AND SANITIZER_MIN_OSX_VERSION VERSION_LESS "10.9")
+  # Mac OS X prior to 10.9 had problems with exporting symbols from
+  # libc++/libc++abi.
+  set(use_cxxabi_default OFF)
+elseif(MSVC)
+  set(use_cxxabi_default OFF)
+else()
+  set(use_cxxabi_default ON)
+endif()
+
+option(SANITIZER_CAN_USE_CXXABI "Sanitizers can use cxxabi" ${use_cxxabi_default})
+pythonize_bool(SANITIZER_CAN_USE_CXXABI)
+
 #================================
 # Setup Compiler Flags
 #================================
 
 include(config-ix)
 
 if(MSVC)
   # Override any existing /W flags with /W4. This is what LLVM does.  Failing to

 append_list_if(COMPILER_RT_HAS_WD4291_FLAG /wd4291 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4391_FLAG /wd4391 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4722_FLAG /wd4722 SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WD4800_FLAG /wd4800 SANITIZER_COMMON_CFLAGS)
 
 # Warnings to turn off for all libraries, not just sanitizers.
 append_string_if(COMPILER_RT_HAS_WUNUSED_PARAMETER_FLAG -Wno-unused-parameter CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 
-if(APPLE AND SANITIZER_MIN_OSX_VERSION VERSION_LESS "10.9")
-  # Mac OS X prior to 10.9 had problems with exporting symbols from
-  # libc++/libc++abi.
-  set(SANITIZER_CAN_USE_CXXABI FALSE)
-elseif(MSVC)
-  set(SANITIZER_CAN_USE_CXXABI FALSE)
-else()
-  set(SANITIZER_CAN_USE_CXXABI TRUE)
-endif()
-pythonize_bool(SANITIZER_CAN_USE_CXXABI)
-
 add_subdirectory(include)
 
 set(COMPILER_RT_LIBCXX_PATH ${LLVM_MAIN_SRC_DIR}/projects/libcxx)
 if(EXISTS ${COMPILER_RT_LIBCXX_PATH}/)
   set(COMPILER_RT_HAS_LIBCXX_SOURCES TRUE)
 else()
   set(COMPILER_RT_HAS_LIBCXX_SOURCES FALSE)
 endif()