[tsan] Disable sysroot flag on FreeBSD

[emaste]

Summary

FreeBSD does not install a number of Clang-provided headers for the compiler in the base system due to incompatibilities between FreeBSD's and Clang's versions. As a workaround do not use --sysroot=. on FreeBSD until this is addressed.

llvm.org/pr26651

[samsonov]

LGTM

compiler-rt/trunk/lib/tsan/CMakeLists.txt

add_dependencies(tsan clang_rt.tsan-${arch}
                           clang_rt.tsan-${arch}-symbols
                           clang_rt.tsan_cxx-${arch}-symbols)
   endforeach()
 endif()
 
 add_dependencies(compiler-rt tsan)
 
 # Make sure that non-platform-specific files don't include any system headers.
-if(COMPILER_RT_HAS_SYSROOT_FLAG)
+# FreeBSD does not install a number of Clang-provided headers for the compiler
+# in the base system due to incompatibilities between FreeBSD's and Clang's
+# versions. As a workaround do not use --sysroot=. on FreeBSD until this is
+# addressed.                                                        
+if(COMPILER_RT_HAS_SYSROOT_FLAG AND NOT CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
   file(GLOB _tsan_generic_sources rtl/tsan*)
   file(GLOB _tsan_platform_sources rtl/tsan*posix* rtl/tsan*mac*
                                    rtl/tsan*linux*)
   list(REMOVE_ITEM _tsan_generic_sources ${_tsan_platform_sources})
   set_source_files_properties(${_tsan_generic_sources}
     PROPERTIES COMPILE_FLAGS "--sysroot=.")
 endif()