enable shared library for orfeotoolbox dependencies

http://www.cmake.org/cmake/help/v3.0/command/if.html says that:

> True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number. False if the constant is 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty string, or ends in the suffix -NOTFOUND.

So it doesn't matter.