COMP: Fix compilation problem: relocation R_X86_64_32 against `a local symbol'

This commits fixes build error occurring when building *without* relying
on BRAINStools Superbuild. In the superbuild case, the -fPIC flag
is SuperBuild.cmake.
Merge pull request #109 from jcfr/fix-code-relocation-build-error

COMP: Fix compilation problem: relocation R_X86_64_32 against `a local s...