Fix command-line option

Author: pitrou

cpp/cmake_modules/SetupCxxFlags.cmake

@@ -169,7 +169,7 @@ endif()
 
 # Disable annoying "performance warning" about int-to-bool conversion
 if ("${COMPILER_FAMILY}" STREQUAL "msvc")
-  set(CXX_COMMON_FLAGS "${CXX_COMMON_FLAGS} /Wd4800")
+  set(CXX_COMMON_FLAGS "${CXX_COMMON_FLAGS} /wd4800")
 endif()
 
 # if build warning flags is set, add to CXX_COMMON_FLAGS