build: use LIBADD, not LDFLAGS, for adding libraries

build: use LIBADD, not LDFLAGS, for adding libraries

Per automake, LDFLAGS is used early in the line, and LIBADD
(libraries) or LDADD (programs) is used late.  On platforms like
cygwin, without lazy linking, this order matters.  Therefore, libtool
commands, -L, and similar should be in LDFLAGS, but -l should be in
L*ADD.

* src/Makefile.am (*_LDFLAGS): Move libraries...
(*_LIBADD): ...to their LIBADD counterpart.