Include libm in statically linked sqlite3 builds

[zanieb]

Looking into #449

[zanieb]

Interestingly... on main we can see the logs

sqlite> checking SQL math functions... enabled
sqlite> checking for library containing ceil...
sqlite> -lm

but with 026c992, we get

sqlite> checking SQL math functions... enabled
sqlite> checking for library containing ceil...
sqlite> none required

and... serialize / deserialize is still not available — I need to take a look at the pkg-config, I think.