It's the automatic formatting in this savannah comments.
after each rule (build:, install:) there must be enter and a tab.
The tab was removed by the comment formatting.
So:

build:
<TAB>scons

install:
<TAB>scons DESTDIR=$(DESTDIR) install

This is very unfortunate.
I was considering contacting them to see if a GNU-compliant build system could be integrated in their code-base when I read this in the README file:

"
We have recently switched from automake/autoconf/make to scons for
building.  Please see the section on Compiling from Source below for
details.
"
[...]

scons is a make/autotools replacement that is very strangely a GNU project (!) which is leading some (hopefully few) projects on a wrong road.

With the limited time at my disposition (and with the GNU scons project being in plain violation of GNU coding standards), I won't support scons directly in sourceinstall in the foreseable future [but I might consider automatic wrapping some day if I learn how to detect such a build system...]

What you can do is to add your own wrapping around that software's build system.
Take the following as a sketch only, since I do not have the possibility of testing on my system: I do not have scons or KDE which are both required.

Install scons. Source installation of scons is a pain in itself. Requires moving stuff around etc [bleah].
Try a binary package, maybe. It requires a recent Python I think.

Then put a simple ./configure script in the software top source directory with something like the following:

-- start --
#! /bin/sh

scons configure
--  end  --

put a simple Makefile in the software top source directory
with something like:

-- start --

build:
        scons

install:
        scons DESTDIR=$(DESTDIR) install
--  end  --

then, from the top source directory run:

$ sourceinstall .

Tell me if this went somewhere, and I hope you can understand
the project position about scons. Some workaround could happen in the future, expecially if I get some help.

Claudio