meson.build: set default help format to html on windows

meson.build: set default help format to html on windows

The build variable DEFAULT_HELP_FORMAT has an appropriate default
('man') set in the code, so there is no need to pass the -Define on
the compiler command-line, unless the build requires a non-standard
value.

In addition, on windows the make build overrides the default help
format to 'html', rather than 'man', in the 'config.mak.uname' file.

In order to suppress the -Define on the C compiler command-line, only
add the -Define to the 'libgit_c_args' variable when the requested
value is not the standard 'man'. In order to override the default value
on windows, add a 'platform' value to the 'default_help_format' combo
option and set it as the default choice. When this option is set to
'platform', use the 'host_machine.system()' method call to determine the
appropriate default value for the host system.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>