Fix as-needed support. Respect system $LDFLAGS

In order for --as-needed ld feature to be used properly libs must
always follow list of object files, otherwise they may be discarded
as unused. More details on --as-needed are here:
http://www.gentoo.org/proj/en/qa/asneeded.xml

Fully respect system $LDFLAGS. Do not force them unless asked to.