Can't compile example from google protocol buffers

Put the library at the end:

g++ -I /home/username/local/include -L /home/username/local/lib main.cpp person.pb.cc -lprotobuf -pthread

From GCC Link Options:

-llibrary
-l library
    Search the library named library when linking. 
    (The second alternative with the library as a separate argument
    is only for POSIX compliance and is not recommended.)

    It makes a difference where in the command you write this option;
    the linker searches and processes libraries and object files in the
    order they are specified.
    Thus, `foo.o -lz bar.o' searches library `z' after file foo.o but
    before bar.o. If bar.o refers to functions in `z', those functions
    may not be loaded.

Also, use -pthread instead of -lpthread as -pthread may set flags for preprocessor and linker.

Library linking flags go at the end of the compiler's arguments:

g++ -I /home/username/local/include -L /home/username/local/lib main.cpp person.pb.cc -lprotobuf -lpthread