Homebrew installed libraries, how do I use them?

I'm a bit confused. I've installed a bunch of libraries (e.g. vtk, gsl,...) that I need for a c++ project for school. I used homebrew to install them (luckily there were formulas available). My question is, does Homebrew link these into /usr/include or something comparable? Or is this something I would need to do manually? Homebrew usually does this kind of stuff for executables, yet I can't seem the find the libraries anywhere other than in the /usr/local/Cellar/... directories. I have a bunch of libraries installed so I can't simply use -L/... for each of them.