Compile with -lpthread

Without this change, `make` returned the following error:

    [...]
    cc -g -O2 -Wall -Wno-unused -o dgamelaunch dgl-common.o ttyrec.o dgamelaunch.o io.o ttyplay.o mygetnstr.o stripgfx.o strlcpy.o strlcat.o setproctitle.o y.tab.o lex.yy.o -lncursesw  -lfl -lutil -lcrypt -lsqlite3 -lrt
    /usr/bin/ld: dgamelaunch.o: undefined reference to symbol 'sem_post@@GLIBC_2.2.5'
    //lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
    collect2: error: ld returned 1 exit status
    make: *** [dgamelaunch] Error 1