nmake: fix install_html_docs target

The nmake rule contains actually two errors:

1. The $< target[1] does not work for regular rules and is
expanded to an empty string after issuing the warning

    NMAKE : warning U4006: special macro undefined : '$<"'

Solution: replace $< by $?

2. The substitution regex is not quoted correctly, which leads
to the following error message by cmd.exe:

    'href' is not recognized as an internal or external command,
    operable program or batch file.

Solution: Quoting arguments for cmd.exe is really a nightmare,
but with the help of the excellent description [2] I was able to
properly quote the regex. Things were complicated by the fact that
a lot of levels of unquoting needed to be considered:

 * perl (windows-makefile.tmpl -> makefile)
 * make (reading the makefile)
 * cmd.exe (executed by make)
 * perl (scanning command line using CommandLineToArgvW())

The fix works, but the regex has become unmaintainable. It would actually
be better to wrap the entire command (including the regex) into a little
perl script which can be called by make directly.

[1] https://docs.microsoft.com/en-us/cpp/build/reference/filename-macros
[2] https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/

Fixes #10648
Fixes #10749

[extended tests]

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10719)
