Do not build .mo files in source directory (bug 14121).

Do not build .mo files in source directory (bug 14121).

Building and installing glibc leaves .mo files (compiled message
translations) behind in the source directory.  Building those files in
the source directory may once have made sense, if they were included
in release tarballs; now that release tarballs are just the output of
"git archive", building any non-checked-in files in the source
directory does not make sense.  This patch changes these files to be
built in the build directory instead.  The realclean rule is changed
to simply adding the .mo files to the "generated" variable, since once
the files are in the build directory it make no sense to exclude them
from normal cleanup rules.

This is necessary but not sufficient to avoid build-many-glibcs.py
needing to copy the glibc source directory.  Its list of files to
touch on checkout to avoid subsequent regeneration (configure,
preconfigure, *-kw.h) is incomplete (missing at least INSTALL,
sysdeps/gnu/errlist.c, posix/testcases.h, posix/ptestcases.h,
locale/C-translit.h, (only regenerated for Hurd builds)
sysdeps/mach/hurd/bits/errno.h, (only regenerated for 32-bit SPARC
builds) sysdeps/sparc/sparc32/{sdiv,udiv,rem,urem}.S) - the existing
list may be sufficient to prevent regeneration that actually changes
the file contents depending on the installed build tools, but not to
ensure there is no regeneration at all - and there might well be other
things writing into the source directory in the course of building and
testing (so needing appropriate testing with read-only source
directories with different timestamp orderings to find and eliminate
all such cases).

Tested for x86_64.

	[BZ #14121]
	* po/Makefile (generated): Add $(ALL_LINGUAS:%=%.mo).
	(%.mo): Change to $(objpfx)%.mo.  Use $(make-target-directory).
	($(mo-installed)): Use $(objpfx)%.mo.
	(realclean): Remove rule.