When you link your libraries, use the -shared flag to create a shared library (.so). For example, to link the tax_input.c example, you type:
gcc -shared -o tax_input.so tax_input.o
If your function depends on third-party or other libraries, you must statically link them.
To see examples of the linking flags in use, review the Makefiles provided in the API directory, sqlmr-sdk/example.