Win: Fix warnings as errors being off for bmesh

bf_bmesh historically always build with the /WX flag
on windows making all warnings errors, somewhere along
the way this has broken for msbuild, ninja still exhibits
the expected behaviour.

The flags are still passed to the target, and I've validated
they are there when the add_library call fires, but they
somehow never make it to the generated msbuild project files.

I suspect this is a cmake bug but I'm seemingly unable
to extract a repro case to file a bug upstream.

Setting the same options target_compile_options seems to work,
I'm not happy about the unexplained nature of the breakage
but this will have to do for now.

source/blender/bmesh/CMakeLists.txt

@@ -182,10 +182,6 @@ set(LIB
   extern_rangetree
 )
 
-if(MSVC AND NOT MSVC_CLANG)
-  string(APPEND CMAKE_C_FLAGS " /WX /wd4101")
-endif()
-
 if(WITH_BULLET)
   list(APPEND INC_SYS
     ${BULLET_INCLUDE_DIRS}
@@ -225,6 +221,10 @@ endif()
 
 blender_add_lib(bf_bmesh "${SRC}" "${INC}" "${INC_SYS}" "${LIB}")
 
+if(MSVC AND NOT MSVC_CLANG)
+  target_compile_options(bf_bmesh PRIVATE /WX /wd4101)
+endif()
+
 if(WITH_GTESTS)
   set(TEST_SRC
     tests/bmesh_core_test.cc