build: Create section for para_filter.

Currently, the construction of $filters, $filter_errlist_objs and
$filter_cmdline_objs is spread out over configure.ac as each test
modifies these variables.

The code in configure.ac becomes easier to read if these variables
are set up in a separate section after all tests have been performed.

This patch also gets rid of filter_ldflags. Due to the previous
cleanups, it only contained -lm which is now added in Makefile.in,
along with all other linker flags for para_filter.