Adding if guards to LIST(REMOVE_DUPLICATES ...) for INCLUDE_DIRS and LIBRARY_DIRS (TriBITS #30)

The rare VUQDemos package in CASL VERA has subpackage that don't define any
libraries.

I don't have a test case for this in TriBITS but adding the if guards fixes
this and I do have tests to ensure that duplicates are removed when there are
INCLUDE_DIRS and LIBRARY_DIRS.

Fixes #30

Build/Test Cases Summary
Enabled Packages: TriBITS, COBRA_TF, CTeuchos
0) MPI_DEBUG => passed: passed=293,notpassed=0 (4.57 min)
1) SERIAL_RELEASE => passed: passed=286,notpassed=0 (3.03 min)
Other local commits for this build/test group: e908f96, c51b77e, 3ea1cd3, 08a9c8f, bfa3094, 7bbcc8d, bda695e, 799c6ee, a12526f, 21910be, b5036bf