Passive->active:

A build-time dependency on <package role="port">lang/go</package>
is added.

Counting the number of variables is not required. It doesn't really help the user, and only gets out of date when the number inevitably changes. So just "There are variables" works... but rearranging the sentence helps:

The build process is controlled by several variables:

or

Variables control the build process:

"Its" works, but "The" seems better.

Please don't use (s) to form optional plurals. It is unnecessary. A plural formed with just "s" is usually valid for any number of packages, including one. Sort of a generalization. However, it's not clear to me when there would be more than one package, or what this does in that case.

As above for "Its" versus "The".

I like this sentence. Have never seen the <application>C</application> before, but that appears to be the right way to do it. Nice!

I admit I lazily adapted the comments at the top of go.mk.

There can be more than one target, there are ports that use it.

Looking at ports using it, it seems that it's some kind of container, like building many .so files and then linking it all together.