build: fix `AM_LDFLAGS` usage (and gcov)

build: fix `AM_LDFLAGS` usage (and gcov)

like the other automake variables, setting `xyz_LDFLAGS` causes
`AM_LDFLAGS` to be ignored for `xyz`.  For some reason I had in my mind
that automake doesn't do this for LDFLAGS, but... it does.  (Which is
consistent with `_CFLAGS` and co.)

So, all the libraries and modules have been ignoring `AM_LDFLAGS` (which
includes `SAN_FLAGS` too).  Set up new `LIB_LDFLAGS` and
`MODULE_LDFLAGS` to handle all of this correctly (and move these bits to
a central location.)

Fixes: #9034
Fixes: 0c4285d77eb ("build: properly split CFLAGS from AC_CFLAGS")
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>