I've tried it applied as usual for patches:
wget https://gitlab.com/a.l.e/scribus/commit/63026e6c1b47dcfa05c223653ecf78fcec9a59ae.diff
mv 63026e6c1b47dcfa05c223653ecf78fcec9a59ae.diff 20191203-ale.diff
Then I edit my build script: as you can see I insert a "read" control to ensure your patch is applied as expected.
I also removed "sed editing" to the CMakeCache.txt.
rm -rf $PRGNAM-$VERSION
tar xvf $CWD/$PRGNAM-$VERSION.tar.?z*
cd $PRGNAM-$VERSION
echo patching...
patch -p1 < $CWD/20191203-ale.diff
echo patch applied, press ENTER
read
chown -R root:root .
find -L . \
\( -perm 777 -o -perm 775 -o -perm 750 -o -perm 711 -o -perm 555 \
-o -perm 511 \) -exec chmod 755 {} \; -o \
\( -perm 666 -o -perm 664 -o -perm 640 -o -perm 600 -o -perm 444 \
-o -perm 440 -o -perm 400 \) -exec chmod 644 {} \;
sed \
-e 's|#!/usr/bin/python|#!/usr/bin/python2|' \
-e 's|#!/usr/bin/env python|#!/usr/bin/env python2|' \
-i scribus/plugins/scriptplugin/{samples,scripts}/*
export CC=clang
export CXX=clang++
export PKG_CONFIG_PATH=/opt/poppler-new/lib64/pkgconfig
cmake . \
-DCMAKE_C_FLAGS="$SLKCFLAGS" \
-DCMAKE_CXX_FLAGS="$SLKCFLAGS" \
-DCMAKE_INSTALL_PREFIX:PATH=/usr \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_SKIP_RPATH=ON \
-DLIB_SUFFIX=${LIBDIRSUFFIX} \
-DWANT_CPP11=1 \
-DRENDER_LIB=CAIRO \
-DWANT_GRAPHICSMAGICK=1
make $NUMJOBS
make install DESTDIR=$PKG
Ok, it builds without any issues!
Great progress.

But after installation, when I launch it, a pop up still appears!
It is the same I said few days ago:
C'è un problema nel caricare 1 di 53 estensioni.
libimportpdf.so
Ciò è causato probabilmente da qualche tipo di anomalia relativa a dipendenze o vecchie estensioni nella cartella d'installazione. Se il problema persiste anche dopo aver svuotato e reinstallato l'estensione, segnalare l'errore su bugs.scribus.net.
This error doesn't prevent to use scribus: if I ignore it and press OK button on the popoup, scribus GUI come up quickly (it seems a bit faster than the old 1.5.4, maybe due to the new poppler, or pherhaps to scribus itself improovments, anyway...).