Give each htmlhandbook target a unique name

Since the CMP0002 warnings have been turned into errors it was
impossible to build kdelibs with -DKDE4_ENABLE_HTMLHANDBOOK:BOOL=TRUE:

"CMake Error at cmake/modules/KDE4Macros.cmake:315 (add_custom_target):
add_custom_target cannot create target "htmlhandbook" because another
target with the same name already exists...."

The bug below is more about the similar error with -DKDE4_BUILD_TESTS
but since it's still open (and I commented there about the htmlhandbook
error ;-) I included it here.

BUG: 351287
REVIEW: 125983