Compiler error: Libraries must be registered in Repository.mk

Historically I've compiled the git clone master in Slackware 14.2 (kde4) 
with the following flags -

> --disable-ccache \
> --disable-gtk3 \
> --enable-kde4 \
> --with-parallelism=5 \
> --without-junit \
> --without-gssapi \
> --without-krb5

never given me a problem

I've now set up a test bed with Slackware Current (kde5) and have tried 
to compile with the following flags

> --disable-ccache \
> --disable-gtk3 \
> --enable-kde5 \
> --with-parallelism=5 \
> --without-junit \
> --without-gssapi \
> --without-krb5 

Almost immediately it fails with

> [info  ALL] currently known libraries are: OGLTrans 
> PresentationMinimizer PresenterScreen abp acc active_native 
> affine_uno_uno analysis animcore avmedia avmediagst basctl basegfx 
> basprov bib binaryurp bootstrap cached1 cairocanvas calc canvasfactory 
> canvastools chartcontroller chartcore chartopengl clew clucene cmdmail 
> collator_data comphelper configmgr cppcanvas cppu cppuhelper ctl cui 
> date dba dbahsql dbase dbaxml dbmm dbp dbpool2 dbtools dbu deployment 
> deploymentgui deploymentmisc desktop_detector desktopbe1 dict_ja 
> dict_zh dlgprov drawinglayer editeng embobj emboleobj emfio epoxy 
> evtatt expwrap file filterconfig firebird_sdbc flash flat for forui 
> fps_office frm fsstorage fwe fwi fwk fwl fwm gcc3_uno gie 
> graphicfilter guesslang helplinker hsqldb hwp hyphen i18nlangtag 
> i18npool i18nsearch i18nutil icg index_data introspection invocadapt 
> invocation io java_uno javaloader javavm jdbc jpipe juh juhx jvmaccess 
> jvmfwk kde5be1 ldapbe2 lng lnth localebe1 localedata_en localedata_es 
> localedata_euro localedata_others log log_uno_uno losessioninstall 
> lwpft mcnttype migrationoo2 migrationoo3 mork mozbootstrap msfilter 
> msforms msword mtfrenderer mysql mysqlc mysqlcppconn namingservice 
> neon numbertext odbc odfflatxml offacc officebean oglcanvas oox opencl 
> package2 passive_native passwordcontainer pcr pdffilter pdfimport 
> pdfium postgresql-sdbc postgresql-sdbc-impl pricing protocolhandler 
> proxyfac purpenvhelper pythonloader pyuno pyuno_wrapper recentfile 
> reflection reg rpt rptui rptxml sal sal_textenc salhelper sax sb sc 
> scd scfilt scn scqahelper scriptframe scui sd sdbc2 sdbt sdd sdfilt 
> sdui sfx simplecanvas slideshow sm smd smoketest sofficeapp solver sot 
> spell spl srtrs1 stocservices storagefd store stringresource 
> subsequenttest svgfilter svgio svl svt svx svxcore sw swd swui syssh 
> t602filter test test-setupvcl testtools_bridgetest 
> testtools_bridgetest-common testtools_constructors testtools_cppobj 
> textconv_dict textconversiondlgs textfd tk tl ucb1 ucbhelper ucpchelp1 
> ucpcmis1 ucpdav1 ucpexpand1 ucpext ucpfile1 ucpftp1 ucpgio1 ucphier1 
> ucpimage ucppkg1 ucptdoc1 unobootstrapprotector unoexceptionprotector 
> unoidl unopkgapp unordf unotest unowinreg unoxml unsafe_uno_uno 
> updatefeed utl uui uuresolver vbaevents vbahelper vbaobj vbaswobj vcl 
> vclbootstrapprotector vclcanvas vclplug_gen vclplug_gtk vclplug_kde5 
> wpftcalc wpftdraw wpftimpress wpftqahelper wpftwriter writer 
> writerfilter writerperfect xmlfa xmlfd xmlreader xmlscript xmlsecurity 
> xo xof xsec_xmlsec xsltdlg xsltfilter xstor
> /home/alex/master/vcl/Library_vclplug_kde5.mk:36: *** Cannot link 
> against library/libraries vclplug_qt5. Libraries must be registered in 
> Repository.mk or RepositoryExternal.mk.  Stop.
> make: *** [Makefile:286: build] Error 2

I've also tried compiling on Kubuntu 17.10 and get the same error.

Can anyone advise on how this can be fixed.

Alex