Fixes check for atomics.

The upstream currently does not have checks for atomics in v6.17.3.
However, since v6.22.1, rocksdb includes checks similar/identical to 
files/rocksdb-6.14.6-libatomic.patch, which is superseded by this new patch. 
See: https://github.com/facebook/rocksdb/commit/47b424f4bd51078591e674ff936de5a270530ce2
Once the upstream adopts new methods for checking, this patch may be removed after testing.

https://bugs.gentoo.org/834855

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -289,6 +289,7 @@ else()
 endif()
 
 include(CheckCXXSourceCompiles)
+set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
 if(NOT MSVC)
   set(CMAKE_REQUIRED_FLAGS "-msse4.2 -mpclmul")
 endif()
@@ -305,7 +306,6 @@ int main() {
   auto d = _mm_cvtsi128_si64(c);
 }
 " HAVE_SSE42)
-unset(CMAKE_REQUIRED_FLAGS)
 if(HAVE_SSE42)
   add_definitions(-DHAVE_SSE42)
   add_definitions(-DHAVE_PCLMUL)
@@ -313,6 +313,37 @@ elseif(FORCE_SSE42)
   message(FATAL_ERROR "FORCE_SSE42=ON but unable to compile with SSE4.2 enabled")
 endif()
 
+# Check if -latomic is required or not
+if (NOT MSVC)
+  set(CMAKE_REQUIRED_FLAGS "--std=c++11")
+  set(ATOMIC_TEST_SOURCE "
+	#include <atomic>
+	std::atomic<int> x;
+	std::atomic<short> y;
+	std::atomic<char> z;
+        std::atomic<long long> w;
+	int main() {
+		++z;
+		++y;
+                ++w;
+		return ++x;
+	}")
+  CHECK_CXX_SOURCE_COMPILES("${ATOMIC_TEST_SOURCE}" BUILTIN_ATOMIC)
+  if (NOT BUILTIN_ATOMIC)
+    set(CMAKE_REQUIRED_LIBRARIES atomic)
+    CHECK_CXX_SOURCE_COMPILES("${ATOMIC_TEST_SOURCE}" ATOMICS_REQUIRE_LIBATOMIC)
+    unset(CMAKE_REQUIRED_LIBRARIES)
+    if (ATOMICS_REQUIRE_LIBATOMIC)
+      list(APPEND THIRDPARTY_LIBS atomic)
+    else()
+      message(FATAL_ERROR "Host compiler must support std::atomic!")
+    endif()
+  endif()
+endif()
+
+# Reset the required flags
+set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+
 CHECK_CXX_SOURCE_COMPILES("
 #if defined(_MSC_VER) && !defined(__thread)
 #define __thread __declspec(thread)
@@ -1354,3 +1385,4 @@ option(WITH_EXAMPLES "build with examples" OFF)
 if(WITH_EXAMPLES)
   add_subdirectory(examples)
 endif()
+