Merge pull request dropbox#96 from jnguyen75/fix-windows-compile

Fix MSVC Compilation: Add MSVC warning flags

CMakeLists.txt

@@ -15,7 +15,18 @@ endif()
 add_library(json11 json11.cpp)
 target_include_directories(json11 PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
 target_compile_options(json11
-  PRIVATE -fPIC -fno-rtti -fno-exceptions -Wall -Wextra -Werror)
+  PRIVATE -fPIC -fno-rtti -fno-exceptions -Wall)
+
+# Set warning flags, which may vary per platform
+include(CheckCXXCompilerFlag)
+set(_possible_warnings_flags /W4 /WX -Wextra -Werror)
+foreach(_warning_flag in ${_possible_warnings_flags})
+  CHECK_CXX_COMPILER_FLAG(_warning_flag _flag_supported)
+  if(${_flag_supported})
+    target_compile_options(json11 PRIVATE ${_warning_flag})
+  endif()
+endforeach()
+
 configure_file("json11.pc.in" "json11.pc" @ONLY)
 
 if (JSON11_BUILD_TESTS)