XERCESC-2208: Use cstdint and standard integer types

[rleigh-codelibre]

• Unconditionally use <cstdint>. Also use <cstddef> and <cwchar>.
• Remove Autoconf and CMake integer checks, along with some other unused header checks and defines present in Xerces_autoconf_config.hpp
• Move constant type definitions out of Xerces_autoconf_config.hpp into XercesDefs.hpp
• UTF16Ch and UCS4Ch are typedefs for char16_t and char32_t, so are now using the language types specifically intended for the purpose
• XSValue now uses fixed-size integer types, so its behaviour will be the same across all platforms.

[rleigh-codelibre]

Tested on additional 32-bit platform (Debian 10, GCC 8.3) and it builds without error, and XSValue and all the other tests are passing.

[boris-kolpackov]

I tend to think this (and other similar changes) can only go into 4.0.0. Should we make master explicitly 4-series?

[rleigh-codelibre]

Hi Boris,

I think that's a good idea, and I think that Scott would also prefer it as well from the current discussion.

Thanks,
Roger

[scantor]

I don't have any particular preference, but given that it's unlikely everything you might want to do will be possible without an API change, I'm sure it's simplest.

[rleigh-codelibre]

I've done the version update to 4.0.0 on the master branch. If you're happy to proceed with this change, it should be OK to go in.

[rleigh-codelibre]

Rebased against current master (no source changes). If there are no objections to merging, will merge later today.