[autotools] gtk-doc.m4 - cross compile support

[Alon Bar-Lev <alon barlev gmail com>]

Hello,

gtk-doc.m4[1] is using pkg-config to locate the gtk-doc tool. This is
working for native build, however, when cross compiling the gtk-doc
tool should be installed on the build and not host, hence it is not
available at pkg-config.

It seems like pkg-config is just used for testing version
compatibility, this can be replaced by executing a tool with version
argument or when cross compiling require an environment variable with
version of the tool to bypass.

What do you think the best way to support cross compile?

Thanks,
Alon

[1] https://github.com/GNOME/gtk-doc/blob/master/buildsystems/autotools/gtk-doc.m4