The libraries that PIC16 port depends on are the microcontroller device libraries which contain the symbol definitions for the microcontroller special function registers. These libraries have the format pic18fxxxx.lib, where xxxx is the microcontroller identification number. The specific library is selected automatically by the compiler at link stage according to the selected device.
Libraries are created with gplib which is part of the gputils package http://sourceforge.net/projects/gputils.
Before using SDCC/pic16 there are some libraries that need to be compiled. This process is not done automatically by SDCC since not all users use SDCC for pic16 projects. So each user should compile the libraries separately.
The steps to compile the pic16 libraries under Linux and Mac OS X are:
./configure
make
cd ..
make model-pic16
su -c 'make install' # install the libraries, you need the root password
su -c 'make install' # install the headers, you need the root password