8-byte atomics check in CMakeLists is broken: it does not pass -latomic flag when it is required

[barracuda156]

It appears from CMakeLists that there is a check for atomics support and a need for libatomic, but it does not do anything. The required flag is not passed, and linking fails:

Undefined symbols:
  "___atomic_fetch_add_8", referenced from:
      __ZN14HttpDataStreamC4Ev in HttpDataStream.cpp.o
ld: symbol(s) not found
collect2: error: ld returned 1 exit status

Configure does something crazy :)

-- Performing Test HAVE_CXX_ATOMICS64_WITHOUT_LIB
-- Performing Test HAVE_CXX_ATOMICS64_WITHOUT_LIB - Failed
-- Looking for __atomic_load_8 in atomic
-- Looking for __atomic_load_8 in atomic - found
-- Performing Test HAVE_CXX_ATOMICS64_WITH_LIB
-- Performing Test HAVE_CXX_ATOMICS64_WITH_LIB - Success
-- Performing Test LLVM_HAS_ATOMICS
-- Performing Test LLVM_HAS_ATOMICS - Success

So despite HAVE_CXX_ATOMICS64_WITHOUT_LIB - Failed, it does not pass -latomic. I have no idea why and what it tried to check for LLVM, when GCC is used and compiler-rt is not even installed.