Fixed compile error on 32 bit platforms related to atomic ints.

Due to an error in CMake code, vtkAtomicInt.cxx was not being
compiled and was causing missing symbols on 32 bit platforms.
Also fixed export macro for atomic int implementation classes.

Change-Id: I2d7666944cd6180a609b404869ed8bcc86edcf71
Merge topic 'atomic-32bit-fix' into master

763d8ae8 Fixed compile error on 32 bit platforms related to atomic ints.