Which version of Ubuntu do you use? Hopefully 14.04, as older versions had only SFML 1.6 in the official repos, since 14.04 Update 1, SFML 2.1 is in the repos.
When installing SFML via apt-get, it gets installed to /usr/, so normally you do not need to specify an additional include path for compiler and linker, as the gcc toolchain automatically searches in /usr/
What do you mean by installing it "manually"?
The normal manual installation method of SFML is by cloning from GitHub, and then building it yourself. This way it gets installed to /usr/local/, where the gcc toolchain also searches per default on Ubuntu 14.04.
I recommend you build it yourself from GitHub, cause the 2.1 version contains some linux specific bugs, which are ironed out in the git commits.
You can find a guide to this
here