Introduce FAIL_ON_WARNINGS CMake variable (default ON)

Summary:
If FAIL_ON_WARNINGS is OFF, compilation will succeed if there were compile
warnings.
Closes https://github.com/facebook/rocksdb/pull/1446

Differential Revision: D4107825

Pulled By: yiwu-arbug

fbshipit-source-id: 4ce6e05

CMakeLists.txt

@@ -86,14 +86,23 @@ target_include_directories(build_version PRIVATE
 if(WIN32)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Zi /nologo  /EHsc /GS /Gd /GR /GF /fp:precise /Zc:wchar_t /Zc:forScope /errorReport:queue")
 
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /FC /d2Zi+ /W3 /WX /wd4127 /wd4800 /wd4996 /wd4351")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /FC /d2Zi+ /W3 /wd4127 /wd4800 /wd4996 /wd4351")
 else()
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -W -Wextra -Wall -Werror")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -W -Wextra -Wall")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wsign-compare -Wshadow -Wno-unused-parameter -Wno-unused-variable -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-omit-frame-pointer -momit-leaf-frame-pointer")
 endif()
 
+option(FAIL_ON_WARNINGS "Treat compile warnings as errors" ON)
+if(FAIL_ON_WARNINGS)
+  if(MSVC)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /WX")
+  else() # assume GCC
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
+  endif()
+endif()
+
 option(WITH_ASAN "build with ASAN" OFF)
 if(WITH_ASAN)
   add_definitions(-DROCKSDB_TSAN_RUN)