Switch from gtk3 to zenity for osdialog on Linux.

Makefile

@@ -27,8 +27,7 @@ ifdef ARCH_LIN
 	SED := sed -i
 	TARGET := libRack.so
 
-	SOURCES += dep/osdialog/osdialog_gtk3.c
-build/dep/osdialog/osdialog_gtk3.c.o: FLAGS += $(shell pkg-config --cflags gtk+-3.0)
+	SOURCES += dep/osdialog/osdialog_zenity.c
 
 	# This prevents static variables in the DSO (dynamic shared object) from being preserved after dlclose().
 	# I don't really understand the side effects (see GCC manual), but so far tests are positive.
@@ -39,7 +38,6 @@ build/dep/osdialog/osdialog_gtk3.c.o: FLAGS += $(shell pkg-config --cflags gtk+-
 	LDFLAGS += dep/lib/libGLEW.a dep/lib/libglfw3.a dep/lib/libjansson.a dep/lib/libcurl.a dep/lib/libssl.a dep/lib/libcrypto.a dep/lib/libarchive.a dep/lib/libzstd.a dep/lib/libspeexdsp.a dep/lib/libsamplerate.a dep/lib/librtmidi.a dep/lib/librtaudio.a
 	LDFLAGS += -Wl,--no-whole-archive
 	LDFLAGS += -lpthread -lGL -ldl -lX11 -lasound -ljack -lpulse -lpulse-simple
-	LDFLAGS += $(shell pkg-config --libs gtk+-3.0)
 
 	STANDALONE_TARGET := Rack
 	STANDALONE_LDFLAGS += -static-libstdc++ -static-libgcc