Update the CheckAtomic CMake module to check for __atomic_fetch_add_4 instead...

Update the CheckAtomic CMake module to check for __atomic_fetch_add_4 instead of __atomic_is_lock_free

Summary:
When building with clang, the build fails with:

```
-- Looking for __atomic_is_lock_free in atomic
-- Looking for __atomic_is_lock_free in atomic - not found
CMake Error at cmake/CheckAtomic.cmake:90 (message):
  Host compiler appears to require libatomic, but cannot find it.
Call Stack (most recent call first):
  CMakeLists.txt:75 (include)
```

And the error is:

```
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:7:3: error: conflicting types for '__atomic_is_lock_free'
  CHECK_FUNCTION_EXISTS(void);
  ^
<command line>:1:31: note: expanded from here
#define CHECK_FUNCTION_EXISTS __atomic_is_lock_free
                              ^
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:7:3: note: '__atomic_is_lock_free' is a builtin with type 'int (unsigned long, const volatile void *)'
<command line>:1:31: note: expanded from here
#define CHECK_FUNCTION_EXISTS __atomic_is_lock_free
                              ^
/usr/share/cmake-3.10/Modules/CheckFunctionExists.c:17:25: error: too few arguments to function call, expected 2, have 0
  CHECK_FUNCTION_EXISTS();
  ~~~~~~~~~~~~~~~~~~~~~ ^
```

LLVM's CheckAtomic (https://fburl.com/bk14shjt) uses `__atomic_fetch_add_4` so I'm modifying the configs to use it as well to check for the existence of the library.

Reviewed By: yfeldblum

Differential Revision: D19497168

fbshipit-source-id: 64f77487efd16dba49055f6c4cb1cdd0fc4ae6da

build/fbcode_builder/CMake/CheckAtomic.cmake

+# University of Illinois/NCSA
+# Open Source License
+#
+# Copyright (c) 2003-2017 University of Illinois at Urbana-Champaign.
+# All rights reserved.
+#
+# Developed by:
+#
+#     LLVM Team
+#
+#     University of Illinois at Urbana-Champaign
+#
+#     http://llvm.org
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy of
+# this software and associated documentation files (the "Software"), to deal with
+# the Software without restriction, including without limitation the rights to
+# use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+# of the Software, and to permit persons to whom the Software is furnished to do
+# so, subject to the following conditions:
+#
+#     * Redistributions of source code must retain the above copyright notice,
+#       this list of conditions and the following disclaimers.
+#
+#     * Redistributions in binary form must reproduce the above copyright notice,
+#       this list of conditions and the following disclaimers in the
+#       documentation and/or other materials provided with the distribution.
+#
+#     * Neither the names of the LLVM Team, University of Illinois at
+#       Urbana-Champaign, nor the names of its contributors may be used to
+#       endorse or promote products derived from this Software without specific
+#       prior written permission.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+# FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
+# CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE
+# SOFTWARE.
+
+include(CheckCXXSourceCompiles)
+
+# Sometimes linking against libatomic is required for atomic ops, if
+# the platform doesn't support lock-free atomics.
+
+function(check_working_cxx_atomics varname)
+  set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+  get_directory_property(compile_options COMPILE_OPTIONS)
+  set(CMAKE_REQUIRED_FLAGS ${compile_options})
+  CHECK_CXX_SOURCE_COMPILES("
+#include <atomic>
+int main() {
+  struct Test { int val; };
+  std::atomic<Test> s;
+  s.is_lock_free();
+}" ${varname})
+  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+endfunction(check_working_cxx_atomics)
+
+
+if(NOT DEFINED PROXYGEN_COMPILER_IS_GCC_COMPATIBLE)
+  if(CMAKE_COMPILER_IS_GNUCXX)
+    set(PROXYGEN_COMPILER_IS_GCC_COMPATIBLE ON)
+  elseif(MSVC)
+    set(PROXYGEN_COMPILER_IS_GCC_COMPATIBLE OFF)
+  elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    set(PROXYGEN_COMPILER_IS_GCC_COMPATIBLE ON)
+  elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Intel")
+    set(PROXYGEN_COMPILER_IS_GCC_COMPATIBLE ON)
+  endif()
+endif()
+
+# This isn't necessary on MSVC, so avoid command-line switch annoyance
+# by only running on GCC-like hosts.
+if(PROXYGEN_COMPILER_IS_GCC_COMPATIBLE)
+  # First check if atomics work without the library.
+  check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITHOUT_LIB)
+  # If not, check if the library exists, and atomics work with it.
+  if(NOT HAVE_CXX_ATOMICS_WITHOUT_LIB)
+    check_library_exists(atomic __atomic_fetch_add_4 "" HAVE_LIBATOMIC)
+    if(HAVE_LIBATOMIC)
+      list(APPEND CMAKE_REQUIRED_LIBRARIES "atomic")
+      check_working_cxx_atomics(HAVE_CXX_ATOMICS_WITH_LIB)
+      if (NOT HAVE_CXX_ATOMICS_WITH_LIB)
+	message(FATAL_ERROR "Host compiler must support std::atomic!")
+      endif()
+      list(APPEND CMAKE_CXX_STANDARD_LIBRARIES -latomic)
+    else()
+      message(FATAL_ERROR "Host compiler appears to require libatomic, but cannot find it.")
+    endif()
+  endif()
+endif()