www/htmldoc-x11: Update to 1.9.20

# Changes in HTMLDOC v1.9.20

- Fixed a regression that caused spaces to disappear between some words
  (Issue #533)
- Fixed resolution of relative links within a document (Issue #534)


# Changes in HTMLDOC v1.9.19

- Security: Fixed an issue with the `file_basename` implementation (Issue #532)
- Added support for 'file' method in links (Issue #512)
- Updated HTML and header/footer code to use a string pool to simplify memory
  management and fix potential double-free bugs.
- Updated configure script to look for zlib with pkg-config (Issue #519)
- Updated markdown support code to mmd.
- Fixed hyperlinks to subfolders (Issue #525)
- Fixed export of UTF-8 HTML (Issue #526)
- Fixed handling of whitespace-only nodes (Issue #528)
- Fixed handling of tabs in PRE nodes (Issue #529)
- Fixed case sensitivity of link targets (Issue #530)


# Changes in HTMLDOC v1.9.18

- Fixed table rendering when there are missing `</tr>` (Issue #494)
- Fixed support for links of the form "filename.html#anchor" in PDF output
  (Issue #514)
- Fixed `--header1` support for web page output (Issue #515)
- Fixed markdown emphasized, strong, and struck-through text (Issue 517)


# Changes in HTMLDOC v1.9.17

- Added new `--pre-indent` option to control indentation of pre-formatted text
  (Issue #505)
- Now link to CUPS library instead of embedding its HTTP code.
- Updated PostScript and PDF date/time information to use UTC (Issue #490)
- Fixed multiple conversions of UTF-8 HTML files from the GUI (Issue #496)
- Fixed a compile bug on Solaris (Issue #498)
- Fixed a markdown parsing bug (Issue #503)
- Fixed a relative URL handling bug (Issue #507)
- Fixed a crash bug with bad title images (Issue #510)
- Fixed some minor CodeQL warnings.
