libgcc of the gcc being built, not the gcc building it.
* Only include a directory in the rpath of an executable/library if it
is actually used. Before, the `/lib' directory of every build input
was added to the rpath, causing many unnecessary retained
dependencies. For instance, Perl has a `/lib' directory, but most
applications whose build process uses Perl don't actually link
against Perl. (Also added a test for this.)
* After building glibc, remove glibcbug, to prevent a retained
dependency on gcc.
* Add a newline after `building X' in GNU Make.
svn path=/nixpkgs/trunk/; revision=911
