This bug is triggered by Debian's build system. When using upstream source, libxml2 is detected via pkg-config.
That said, the underlying configure.ac code is overly complex. While xml2-config (and xslt-config) still exist, pkg-config is far easier to use for detection. I'm happy to patch the source. Are you happy to replace all calls to xml2-config (and xslt-config) with the PKG_CHECK_MODULES macro? Given that it's 2020, and many foo-config scripts have or are being deprecated/removed, it's probably time to do so in raptor2 as well.
