Use pkg-config to discover Qt (pkg-config is already a requirement anyway). * build-aux/common.mk (do_subst_config_vals): Replace QT_INCDIR and QT_LIBDIR with QT_CPPFLAGS, QT_LDFLAGS and QT_LIBS. * configure.ac: Likewise and use pkg-config to discover the new variables. * libgui/Makefile.am (QT_INCDIR, QT_LIBDIR, QT_LIBS, QT_LDFLAGS): Delete variable definitions. * libgui/link-deps.mk (LIBOCTGUI_LINK_DEPS): Use automake @QT_LIBS@. (LIBOCTGUI_LIK_OPTS): Use automake @QT_LDFLAGS@. * libgui/qterminal-module.mk (qterminal_libqterminal_la_CPPFLAGS): Use automake @QT_CPPFLAGS@ instead of QT_INCDIR-based include paths. (MOC_CPPFLAGS): Define Q_OS_WIN32 instead of WIN32. * libgui/src/module.mk (src_libgui_src_la_CPPFLAGS): Use automake @QT_CPPFLAGS@ instead of QT_INCDIR-based include paths. * libinterp/interpfcn/toplev.cc (Foctave_config_info): Replace QT_INCDIR and QT_LIBDIR with QT_CPPFLAGS, QT_LDFLAGS and QT_LIBS. * libinterp/oct-conf.in.h: Replace OCTAVE_CONF_QT_INCDIR and OCTAVE_CONF_QT_LIBDIR with OCTAVE_CONF_QT_CPPFLAGS, OCTAVE_CONF_QT_LDFLAGS and OCTAVE_CONF_QT_LIBS. * m4/acinclude.m4 (OCTAVE_CHECK_FUNC_FINDFIRST_MODERN, OCTAVE_CHECK_FUNC_SETPLACEHOLDERTEXT): Use QT_CPPFLAGS instead of QT_INCDIR.

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