Switch to GTK+3 for osdialog.

Makefile

@@ -25,8 +25,8 @@ LDFLAGS += -shared
 ifdef ARCH_LIN
 	TARGET := libRack.so
 
-	SOURCES += dep/osdialog/osdialog_gtk2.c
-build/dep/osdialog/osdialog_gtk2.c.o: FLAGS += $(shell pkg-config --cflags gtk+-2.0)
+	SOURCES += dep/osdialog/osdialog_gtk3.c
+build/dep/osdialog/osdialog_gtk3.c.o: FLAGS += $(shell pkg-config --cflags gtk+-3.0)
 
 	# 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.
@@ -36,7 +36,7 @@ build/dep/osdialog/osdialog_gtk2.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/libzip.a dep/lib/libz.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
-	LDFLAGS += $(shell pkg-config --libs gtk+-2.0)
+	LDFLAGS += $(shell pkg-config --libs gtk+-3.0)
 
 	STANDALONE_TARGET := Rack
 	STANDALONE_LDFLAGS += -Wl,-rpath=.