build: build libosmotrau.la in the src directory.

This spares us another recursion, and allows a much higher
parallelisation, since trau is only one source unit and one standalone
library.

Incidentally, also make sure that the version specification for
libosmoabis is applied.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
3 files changed