Build the configuration GUI against Qt6 instead of Qt5.

Qt5 has been removed from the Gentoo tree. The C++ sources already compile
unmodified against Qt6; only the build system hardcodes Qt5 library names, the
Qt5 include path and the unprefixed moc/uic/pyuic5 binaries. Make all of those
overridable and default them to Qt6.

Also fix a latent dependency bug: dialog.cpp includes device_selector.hpp,
which includes the generated ui_device_selector.hpp, but only
device_selector.cpp declared that dependency. This never bit upstream because
the generated files are checked into the repository; it fails as soon as they
are regenerated. Depend on the whole generated set from every object instead.

diff --git a/new_gui/Makefile b/new_gui/Makefile
index 2ccaa5c..11f1849 100644
--- a/new_gui/Makefile
+++ b/new_gui/Makefile
@@ -6,8 +6,14 @@ ifneq ($(M),)
 ARCH_FLAG = -m$(M)
 endif
 
+MOC ?= moc
+UIC ?= uic
+PYUIC ?= pyuic6
+
+QT_LIBS ?= -lQt6Core -lQt6Widgets -lQt6Gui
+
 CXXFLAGS = -fpic -std=c++20 -ggdb $(ARCH_FLAG)
-LDFLAGS = $(ARCH_FLAG) -lQt5Core -lQt5Widgets -lQt5Gui -fpic -lpipewire-0.3
+LDFLAGS = $(ARCH_FLAG) $(QT_LIBS) -fpic -lpipewire-0.3
 
 BUILD_DIR = ../build$(M)
 COMMON_OBJS := dialog.o device_selector.o device_chooser.o
@@ -25,18 +31,19 @@ $(BUILD_DIR)/libpwasio_gui.so: $(call objs,$(COMMON_OBJS) gui.o)
 	$(CXX) -o $@ -shared $(LDFLAGS) $^ $(BUILD_DIR)/$(ASIO_DLL_MODULE).so
 endif
 
-device_selector.cpp: ui_device_selector.hpp
-dialog.cpp: ui_dialog.hpp
 
-QT_CFLAGS = -isystem /usr/include/qt
+QT_CFLAGS ?= -isystem /usr/include/qt6 \
+	-isystem /usr/include/qt6/QtCore \
+	-isystem /usr/include/qt6/QtGui \
+	-isystem /usr/include/qt6/QtWidgets
 PW_CFLAGS = -isystem /usr/include/pipewire-0.3 -isystem /usr/include/spa-0.2
 
-dialog.cpp: dialog.moc
-device_selector.cpp: device_selector.moc
-device_chooser.cpp: device_chooser.moc
-gui.cpp: gui_priv.moc
+# Every translation unit pulls in the generated headers transitively, so make
+# them all depend on the full set rather than guessing per-file edges.
+GENERATED = ui_dialog.hpp ui_device_selector.hpp \
+	dialog.moc device_selector.moc device_chooser.moc gui_priv.moc
 
-$(BUILD_DIR)/%.o: %.cpp
+$(BUILD_DIR)/%.o: %.cpp $(GENERATED)
 	@$(shell mkdir -p $(BUILD_DIR))
 	$(CXX) -c -o $@ $(CXXFLAGS) $(QT_CFLAGS) $(PW_CFLAGS) $<
 
@@ -45,10 +52,10 @@ $(BUILD_DIR)/pw_helper.o: ../pw_helper.cpp
 	$(CXX) -c -o $@ $(CXXFLAGS) $(PW_CFLAGS) $<
 
 %.moc: %.hpp
-	moc -o $@ $<
+	$(MOC) -o $@ $<
 
 ui_%.py: %.ui
-	pyuic5 $< -o $@
+	$(PYUIC) $< -o $@
 
 ui_%.hpp: %.ui
-	uic $< -o $@
+	$(UIC) $< -o $@