libc: Bring back warnings to libc.

To disable warnings for the gdtoa sources only, the tweaking of the
CWARNFLAGS has to come _after_ <bsd.lib.mk> is included.

Therefore, it has to be done in libc's Makefile instead of its
Makefile.inc.

Before this commit warnings were completely disabled for libc due
to '-w' passed to every command line.

Also reset WARNS back to 0 since right now, libc doesn't even pass
WARNS=1.
libc: Fix -Wold-style-definition.
libc: Fix -Wunused-variable.
libc: Fix -Wformat.
libc: Fix -Wunused-label.
libc: Fix -Wreturn-type.
libc: Fix -Wempty-body.
libc: Fix -Wmissing-field-initializers.
libc: Fix -Wpointer-arith.
libc: Fix -Wunused-parameter.
lock(1): Use %jd for printing intmax_t.