build/msvc: Workaround for compiler optimization bug

src/nvim/option.c

@@ -2436,6 +2436,11 @@ static bool valid_filetype(char_u *val)
   return true;
 }
 
+#ifdef _MSC_VER
+// MSVC optimizations are disabled for this function because it
+// incorrectly generates an empty string for SHM_ALL.
+#pragma optimize("", off)
+#endif
 /*
  * Handle string options that need some action to perform when changed.
  * Returns NULL for success, or an error message for an error.
@@ -3305,6 +3310,9 @@ did_set_string_option (
 
   return errmsg;
 }
+#ifdef _MSC_VER
+#pragma optimize("", on)
+#endif
 
 /*
  * Simple int comparison function for use with qsort()