Use /W4 in format-test and only if FMT_EXTRA_TESTS is set

as (1) we don't want to force /W4 on users of the library and (2) some of
the warnings only show up when the formatting functions are used.

CMakeLists.txt

@@ -83,8 +83,6 @@ add_library(format ${FMT_SOURCES})
 if (CMAKE_COMPILER_IS_GNUCXX)
   set_target_properties(format PROPERTIES COMPILE_FLAGS
     "-Wall -Wextra -Wshadow -pedantic")
-elseif (MSVC)
-  set_target_properties(format PROPERTIES COMPILE_FLAGS /W4)
 endif ()
 
 # If FMT_EXTRA_TESTS is TRUE, then test compilation with both -std=c++11