Remove "all"

s/one/a single/

"What it basically does" is not needed. Better to just name it and use the present tense. Also, the subdirectory name should be in <filename> tags when being referred to as a directory (rather than a value to be set).

<varname>GH_SUBDIR</varname> makes sure that
<filename>ext</filename> exists, but that
<filename>ext/icons</filename> does not already exist.  Then it
does this:</para>

Seems like this could be simpler:

Ports with GitHub as an upstream repository sometimes use submodules.

Can probably end the sentence after "information".

s/need to be/must be/

Extra space between "example," and "its".

Break sentence here. s/, each/. Each/

"in fact" is not needed.

Break sentence here:

meaningful information.  For example, here,

Why?

s/have/has/ (singular)

Ah, yes, igor does not report it :-p

Because i said so, 4 lines above, it gives more meaning to it. And I did not want to repeat myself :-)
It's more interesting to use v3.4.0 as it show it is using a "release" and not a specific hash that upstream picked out of a hat.

Also, it has the advantage that if there is a wxsqlite3 port using github and using version 3.4.0, its distfile will be the same, and you get one distfile for two ports and save some space.

Above, it just says "will provide more meaningful information". It doesn't tell how the information given is more meaningful. If it isn't possible to be more specific, probably better to make this imperative rather than a suggestion:

Both can be used
interchangeably, but when a tag is available, use it.</para>