feat: add a simple way to disable <filesystem>

As jwillikers pointed out in #150, there is a case where compiler
defines the corresponding feature test macro of <filesystem> but is
actually not available. The macro is a way to disable the feature
regardless of the status of feature test macro.

toml/parser.hpp

@@ -13,12 +13,14 @@
 #include "types.hpp"
 #include "value.hpp"
 
+#ifndef TOML11_DISABLE_STD_FILESYSTEM
 #ifdef __cpp_lib_filesystem
 #if __has_include(<filesystem>)
 #define TOML11_HAS_STD_FILESYSTEM
 #include <filesystem>
 #endif // has_include(<string_view>)
 #endif // __cpp_lib_filesystem
+#endif // TOML11_DISABLE_STD_FILESYSTEM
 
 namespace toml
 {