Handle Apple and MSVC explictly

Author: vhscampos

libcxx/include/__config

@@ -1023,6 +1023,8 @@ typedef __char32_t char32_t;
 // defined are ignored.
 //
 // picolibc 1.8.9 and newer declare the two functions unconditionally.
+//
+// Microsoft C Runtime and Apple C library do not yet support these functions.
 #  if defined(_LIBCPP_GLIBC_PREREQ)
 #    if defined(__cpp_char8_t) && _LIBCPP_GLIBC_PREREQ(2, 36)
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
@@ -1035,6 +1037,8 @@ typedef __char32_t char32_t;
 #    else
 #      define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
 #    endif
+#  elif defined(__APPLE__) || defined(_MSC_VER)
+#    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 0
 #  else
 #    define _LIBCPP_HAS_C8RTOMB_MBRTOC8 1
 #  endif