build: fix up some compiler flags

build: fix up some compiler flags

Matthias noted that the line:
virt_aa_helper_LDFLAGS = $(WARN_CFLAGS)
looks inconsistent, so I did an audit.

Currently, the set of compiler warning flags passed to gcc as $CC are
equally permitted as the set of linker flags passed to gcc as $LD, so
there was no problem with that usage.  But if we ever get in a
situation where $CC and $LD treat particular flags differently, using
the right variable form will make it easier.

In the process, I spotted a couple of typos that were omitting useful
flags, as well as specifying a -l under the wrong variable.

* acinclude.m4 (LIBVIRT_COMPILE_WARNINGS): Define WARN_LDFLAGS as
an alias for WARN_CFLAGS.
* tools/Makefile.am (virsh_LDFLAGS): Use more canonical spelling.
* proxy/Makefile.am (libvirt_proxy_LDFLAGS): Likewise. Move
library...
(libvirt_proxy_LDADD): ...here.
* src/Makefile.am (virt_aa_helper_LDFLAGS): Use more canonical
spelling of WARN_LDFLAGS.
(libvirt_parthelper_LDFLAGS, libvirt_lxc_LDFLAGS): Likewise.  Use
correct spelling of COVERAGE_LDFLAGS.
Reported by Matthias Bolte.