[rtorrent] Update global CFLAGS/CXXFLAGS/LIBS at the end of configure.ac

[zetafunction]

Changes to CFLAGS/CXXFLAGS/LIBS affect following lines in configure.ac.
As originally written, this would cause random libraries, such as
libtorrent, to be linked into the autoconf test program. Occasionally,
this would cause the things like the pkgconfig checks to fail with
mysterious linker errors (e.g. #76).

Instead, only populate these global flags with the additional flags for
dependencies at the very end, before the invocation of AC_OUTPUT.

This is a followup to #118 and fixes the root cause of random autoconf failures.

config.log for XMLRPC-C check before this change:

configure:18550: checking for XMLRPC-C
configure:18584: g++ -o conftest -g -O2 -g -DDEBUG -I/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -I/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -I/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -I /home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -pthread -std=c++11 -Wl,-rpath,/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib conftest.cpp -lncursesw -lcppunit -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -lcurl -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -ltorrent -lpthread -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -lxmlrpc_server -lxmlrpc -lxmlrpc_xmlparse -lxmlrpc_xmltok -lxmlrpc_util -lpthread >&5
configure:18584: $? = 0
configure:18586: result: ok

config.log for XMLRPC-C check after this change:

configure:18192: checking for XMLRPC-C
configure:18226: g++ -o conftest -g -O2 -g -DDEBUG -I/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -I /home/z/.local/rtorrent/0.9.6-PS-1.2-dev/include -pthread -std=c++11 -Wl,-rpath,/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib conftest.cpp -lpthread -L/home/z/.local/rtorrent/0.9.6-PS-1.2-dev/lib -lxmlrpc_server -lxmlrpc -lxmlrpc_xmlparse -lxmlrpc_xmltok -lxmlrpc_util -lpthread >&5
configure:18226: $? = 0
configure:18228: result: ok

This is a separate patch file from rt-base-cppunit-pkgconfig.patch, since that patch file is no longer needed in rtorrent 0.9.7 and later.

[zetafunction]

I checked that this patch applies successfully against both 0.13.6/0.9.6 and 0.13.7/0.9.7, but I'm happy to change this if you want.