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.

--- a/m4/acinclude.m4
+++ b/m4/acinclude.m4
@@ -153,7 +153,7 @@
     [octave_cv_func_findfirst_modern],
     [AC_LANG_PUSH(C++)
     ac_octave_save_CPPFLAGS="$CPPFLAGS"
-    CPPFLAGS="-I$QT_INCDIR -I$QT_INCDIR/Qt $CPPFLAGS"
+    CPPFLAGS="$QT_CPPFLAGS $CPPFLAGS"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
         #include <Qsci/qsciglobal.h>
         ]], [[
@@ -231,7 +231,7 @@
     [octave_cv_func_setplaceholdertext],
     [AC_LANG_PUSH(C++)
     ac_octave_save_CPPFLAGS="$CPPFLAGS"
-    CPPFLAGS="-I$QT_INCDIR $CPPFLAGS"
+    CPPFLAGS="$QT_CPPFLAGS $CPPFLAGS"
     AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
         #include <Qt/qglobal.h>
         ]], [[