Add -fPIC option for 64 bit linux machine compatibility.

When dart is linked to gazebo on 64 bit Ubuntu machine, the below error is occurred:

:-1: error: /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/libdartd.a(UtilsRotation.cpp.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC

CMakeLists.txt was modified to fix this issue.