Octave 6.0.90 build failure on macOS 10.13 against Qt 5.15.1: link error on missing Qt symbols

Hi folks,

I tried building the Octave 6.0.90 release candidate on macOS 10.13
against the new Qt 5.15.1 LTS release, and it's failing at the link
stage with this error:

libtool: link: clang++ -std=gnu++11 -dynamiclib  -o
libgui/.libs/liboctgui.6.dylib 
libgui/.libs/liboctgui_la-liboctgui-build-info.o  
-Wl,-force_load,libgui/qterminal/.libs/libqterminal.a
-Wl,-force_load,libgui/graphics/.libs/libgraphics.a
-Wl,-force_load,libgui/src/.libs/libgui-src.a 
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/opt/hdf5/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/graphicsmagick/1.3.35/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/fftw/3.3.8_2/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/arpack/3.7.0_5/libexec/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/pcre/8.44/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/opt/qscintilla2-octave-app/lib
libinterp/.libs/liboctinterp.dylib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/opt/openblas/lib
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/gcc/10.2.0/lib/gcc/10/gcc/x86_64-apple-darwin17/10.2.0
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/gcc/10.2.0/lib/gcc/10/gcc/x86_64-apple-darwin17/10.2.0/../../..
-L/Applications/Octave-6.0.90.app/Contents/Resources/usr/opt/freetype/lib
/private/tmp/[email protected]/octave-6.0.90/liboctave/.libs/liboctave.dylib
liboctave/.libs/liboctave.dylib -framework QtNetwork -framework
QtPrintSupport -framework QtHelp -framework QtWidgets -framework QtGui
-framework QtSql -framework QtXml -framework QtCore -lqscintilla2_qt5
-framework QtOpenGL -lhdf5
/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/graphicsmagick/1.3.35/lib/libGraphicsMagick++.dylib
/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/graphicsmagick/1.3.35/lib/libGraphicsMagick.dylib
-llcms2 -lbz2 -lltdl -lpthread -lz -framework OpenGL -lfontconfig
-lfreetype -framework Carbon -lgl2ps -lcurl -lcholmod -lumfpack -lamd
-lcamd -lcolamd -lccolamd -lcxsparse -lsuitesparseconfig
/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/arpack/3.7.0_5/libexec/lib/libarpack.dylib
-lqrupdate -lfftw3_threads -lfftw3 -lfftw3f_threads -lfftw3f -lopenblas
-lreadline -lncurses -lpcre -ldl -lgfortran -lquadmath -liconv -lm 
-pthread
-F/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/qt-octave-app/5.15.1/lib
-F/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/qt-octave-app/5.15.1/lib
-F/Applications/Octave-6.0.90.app/Contents/Resources/usr/opt/qt-octave-app/lib
-Wl,-framework -Wl,CoreFoundation -Wl,-dylib_file
-Wl,/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/[email protected]/6.0.90/lib/octave/6.0.90/liboctave.8.dylib:/private/tmp/[email protected]/octave-6.0.90/liboctave/.libs/liboctave.dylib  
-pthread -framework OpenGL -framework Carbon -framework QtNetwork
-framework QtPrintSupport -framework QtHelp -framework QtWidgets
-framework QtGui -framework QtSql -framework QtXml -framework QtCore
-framework QtOpenGL -install_name 
/Applications/Octave-6.0.90.app/Contents/Resources/usr/Cellar/[email protected]/6.0.90/lib/octave/6.0.90/liboctgui.6.dylib
-compatibility_version 7 -current_version 7.0 -Wl,-single_module
Undefined symbols for architecture x86_64:
  "_class_getClassMethod", referenced from:
      octave::base_qobject::base_qobject(octave::qt_application&) in
libgui-src.a(libgui_src_la-octave-qobject.o)
  "_class_getInstanceMethod", referenced from:
      octave::base_qobject::base_qobject(octave::qt_application&) in
libgui-src.a(libgui_src_la-octave-qobject.o)
  "_objc_getClass", referenced from:
      octave::base_qobject::base_qobject(octave::qt_application&) in
libgui-src.a(libgui_src_la-octave-qobject.o)
  "_objc_msgSend", referenced from:
      octave::base_qobject::base_qobject(octave::qt_application&) in
libgui-src.a(libgui_src_la-octave-qobject.o)
  "_sel_getUid", referenced from:
      octave::base_qobject::base_qobject(octave::qt_application&) in
libgui-src.a(libgui_src_la-octave-qobject.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
make[2]: *** [libgui/liboctgui.la] Error 1
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2


Anyone know what might be going on there?

Build went fine against Qt 5.12 on macOS 10.14. I'm trying other
combinations of OS and Qt version to try to narrow the issue down, but
these builds do take a while...

Cheers,
Andrew