Explicit link to pthread

[bersace]

Fixes:

$ LC_ALL=C make                                                                                                      
[ 50%] Linking CXX executable teller                      
/usr/bin/ld: CMakeFiles/teller.dir/teller.cc.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'         
/usr/bin/ld: /lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line             
collect2: error: ld returned 1 exit status    
make[2]: *** [CMakeFiles/teller.dir/build.make:103: teller] Error 1                                                                                                                                                                     
make[1]: *** [CMakeFiles/Makefile2:95: CMakeFiles/teller.dir/all] Error 2                                                                                                                                                               
make: *** [Makefile:103: all] Error 2

[berthubert]

thank you!