#62 libname conflicts pass undetected
as an alternative to confix's implicit composition of library names, one may specify the library name explicitly, by using one of the following
* for the explicit interface:
LIBRARY(basename='libraryname', ...)
* for the implicit interface:
LIBNAME('libraryname')
using the implicit library name composition, it is unlikely that two libraries with the same name are built - library names are composed of their package names and the path to the directories inside the respective package.
when libraries are explicitly named by the user, then library name conflicts are at the user's choice, and currently go undetected by confix.
fix this and track library names a bit. this can be done regardless of which naming scheme is being used - be it explicit or composed. complain with an error (warning is too less) if conflicts are detected.
