argus: use pkg-config for libtirpc flags

Instead of hardcoding the flags needed for libtirpc, use pkg-config.
The linker flags are added to LIBS rather than LDFLAGS, which is more
correct in case of static builds.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>