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>