setupext: put pkg-config -I, -L, -l locations at the head of the list

This addresses #2622.
The assumption here is that we want the pkg-config info to take
precedence, so its -I and -L output should be prepended, not
appended to the lists used to construct the compiler and linker
commands.  It is less clear whether the -l info also should
be prepended, as it is in the changeset; ideally, it wouldn't
matter, but for some compilers, it might.  If so, -l can
be handled separately from -I and -L.