libc: fix local from-source upgrades

libc: fix local from-source upgrades

Commit git-c38dbb9 inadvertently broke local MINIX3-on-MINIX3 builds,
since its libc changes relied on VFS being upgraded already as well.
As a result, after installing the new libc, networking ceased to work,
leading to curl(1) failing later on in the build process.  This patch
introduces transitional code that is necessary for the build process
to complete, after which it is obsolete again.

Change-Id: I93bf29c01d228e3d7efc7b01befeff682954f54d