C/C++ library for digital modulation
I have created a C++ library that I haven't officially released yet that could be used for modulation.  It is called NimbleDSP.  It doesn't have a function or method called "modulation", but it does have interpolation and supports complex numbers, so it could easily be used to do modulation.

Liquid-dsp appears to be a mature SDR library, and it is used by the government framework RED HAWK that was recently open sourced.  My main problem with the library is that it uses the GPL license (may not be a problem for you) and the interface seems complicated.  Having said that, I looked at it fairly briefly so I could be wrong about that.

There is also SPUC (Signal Processing Using C++) and Aquila.  SPUC is also GPL I believe, but looks to be simpler than liquid-dsp while still being very comprehensive.  Aquila has a very permissive license but has some odd holes in its functionality.