Questions regarding BUILD_DEPENDS definition

Dear all,

I have a question regarding BUILD_DEPENDS.
The manual in bsd.port.mk tells:
# BUILD_DEPENDS - A list of "path:dir[:target]" tuples of other ports this
#                                 package depends to build (between the 
"extract" and
#                                 "build" stages, inclusive).  The test 
done to
#                                 determine the existence of the 
dependency is the
#                                 same as FETCH_DEPENDS.  If the third 
field ("target")
#                                 exists, it will be used instead of 
${DEPENDS_TARGET}.

So if I understood it correctly, it should only be used between the 
extract and build phase but should not be recorded as a dependency if 
the package is installed with pkg?

But it seems that pkg is pulling the BUILD_DEPENDS while installing the 
package.

Clear example is this one:
https://cgit.freebsd.org/ports/tree/devel/gitlab-shell/Makefile#n12

How can I define a build dependency that is required to build the port 
but is not installed with the port?

Thanks.

Gruß
Matthias

-- 

"Programming today is a race between software engineers striving to
build bigger and better idiot-proof programs, and the universe trying to
produce bigger and better idiots. So far, the universe is winning." --
Rich Cook