build with shared libs broken

[michaelelucas]

When building on ubuntu 20.04 LTS with gcc and the following cmake settings caused the build to fail with missing definitions for round in nrt.

cmake -DBUILD_SHARED_LIBS=ON -DCODA_INSTALL_TESTS=OFF -DENABLE_PYTHON=OFF -DENABLE_JARS=OFF ../

Changing nitro/modules/c/nrt/CMakeLists.txt from:

DEPS ${CMAKE_DL_LIBS} config-c++

to:

DEPS ${CMAKE_DL_LIBS} config-c++ m

fixed the issue for me.