Fix underlinking on recent binutils:

> x86_64-pc-linux-gnu-g++ -Wl,-O1 -Wl,--as-needed -Wl,--hash-style=gnu picpuz.o zfuncs.o `pkg-config --libs gtk+-3.0` -o picpuz
> /usr/lib/gcc/x86_64-pc-linux-gnu/4.8.2/../../../../x86_64-pc-linux-gnu/bin/ld: zfuncs.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
> /lib64/libpthread.so.0: error adding symbols: DSO missing from command line
diff --git a/Makefile b/Makefile
index 28638de..700b610 100644
--- a/Makefile
+++ b/Makefile
@@ -19,7 +19,7 @@ MANDIR = $(PREFIX)/share/man/man1
 MENUFILE = $(PREFIX)/share/applications/kornelix-$(PROGRAM).desktop
 
 CXXFLAGS += -c `pkg-config --cflags gtk+-3.0`
-LIBS = `pkg-config --libs gtk+-3.0`
+LIBS = -pthread `pkg-config --libs gtk+-3.0`
 
 $(PROGRAM): $(PROGRAM).o zfuncs.o
 	$(CXX) $(LDFLAGS) $(PROGRAM).o zfuncs.o $(LIBS) -o $(PROGRAM)