* build-aux/connfigure-lib.sh: Split from build-lib.sh
* build-aux/build-lib.sh: Include it.
* build-aux/bootstrap.sh.in: New file.
* configure.sh: Substitute it.
* lib/linux/_getcwd.c: Rename from getcwd.c.
* lib/linux/_open3.c: Rename from open.c.
* lib/posix/getcwd.c: New file with bits from ../linux/getcwd.c.
* lib/posix/open.c: New file with bits ../linux/getcwd.c.
* build-aux/build-lib.sh (libc_SOURCES): Update.