Libraries are often placed under Lesser GPL, called LGPL.
As stated on Wikipedia, in
http://en.wikipedia.org/wiki/LGPL ,
"
The main difference between the GPL and the LGPL is that the latter can be linked to (in the case of a library, 'used by') a non-(L)GPLed program, which may be free software or proprietary software [1]. This non-(L)GPLed program can then be distributed under any chosen terms if it is not a derivative work.
"