examples/lib: work with bundled libhtp

examples/lib: work with bundled libhtp

The simple example Makefile.am was unconditionally including
$(HTP_LDADD) which might be empty resulting in "../.." ending up in the
Makefile causing the build to fail.

Instead, also make HTP_LDADD a conditional, so we can only include it
when actually set, and its only set when libhtp is bundled.

The reason this Makefile needs to include the path components "../.."
is because the HTP_LDADD value is relative to the top level "src/"
directory.