compile libdnds_protocol as shared

Got a problem when building libdnds_protocol:

    Linking CXX shared library libdnds.so
    /usr/bin/ld: protocol/libdnds_protocol.a(DNDSMessage.c.o): relocation R_X86_64_32 against `.rodata.str1.8' can not be used when making a shared object; recompile with -fPIC
    protocol/libdnds_protocol.a: could not read symbols: Bad value
    collect2: error: ld returned 1 exit status

Seems specific to 64 bits build.
After a bit of googling I found that building the library as SHARED solved the problem.

I don't know if it really is a good idea, what's your opinion ?