I had this exact problem and I'd also be very thankful for information on this! For the life of me I couldn't get any C programs to link against the .so files that are installable on the Omega2+. I always had to recompile or cross-compile every library myself (SPI, I2C, ALSA,..) to get the linker to work.
Another interesting thing is that when you do file libonionspi.so it will say ELF 32-bit LSB shared object, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, corrupted section header size.
As a work-around, I've uploaded the recompiled binaries at https://github.com/gamer-cndg/omega2-libs. But it feels like I'm missing something here that would allow for a successful linking with the preinstalled .so files.. Maybe a missing linker flag?