Autotools: move libhtp conditionals to configure

In preparation of the libhtp upgrade, move all libhtp related conditionals
to configure. This allows for one set of build scripts that works regardless
of the presence of a local libhtp dir.
Use relative dir instead of ac_builddir

When generating src/build-info.h the use of ac_builddir was problematic.
There were several cases where it was undefined leading to the absolute
path /src/build-info.h. ./configure should be called from the parent dir
to the actual src dir, so this should work. Make distcheck and normal builds
complete fine.