fix xpSourceListAppend

[smanders]

the enhancements made to xpSourceListAppend #101 introduced an issue for projects that use this function but aren't in a git repo when they're built
(palam, built via internpro)

the repoFiles list is empty and there's a cmake error: list sub-command REMOVE_ITEM requires list to be present.
https://github.com/smanders/externpro/blob/16.09.1/modules/xpfunmac.cmake#L863
because all of the files are untracked and have been removed from the list

[smanders]

solution: put a lot of the xpSourceListAppend() enhancements inside a

if(EXISTS ${CMAKE_SOURCE_DIR}/.git)

conditional (check to see that it's a git repo)

[smanders]

completed with commit to dev branch

[smanders]

also, removed obsolete cmake function xpGenerateCscopeDb() (made obsolete by the xpSourceListAppend() enhancements) with commit to dev branch