build: fix out-of-tree build

When building collectd out of tree, `srcdir` and `builddir` are
different. We ask to search path in `$(top_srcdir)/src` since this is
needed to find `liboconfig/config.h`. Also fix search path for
libcollectdclient where only one header is in `builddir` while the
remaining are in `srcdir`.
Merge pull request #792 from vincentbernat/fix/out-of-tree-build

build: fix out-of-tree build