Makes sure that gtkdoc-scan is also checked during configure
Makes sure that gtkdoc-scan is also checked during configure

This fixes the bug when gtk-doc-tools was not installed, hence
gtkdoc-scan was not available. gtk doc generation is enabled by
default and checks for only valadoc, but it ends up using
gtkdoc-scan too somewhere in the codebase (probably for
extracting docs from header files)

freedesktop-unofficial-mirror/zeitgeist__zeitgeist,freedesktop-unofficial-mirror/zeitgeist__zeitgeist,freedesktop-unofficial-mirror/zeitgeist__zeitgeist,freedesktop-unofficial-mirror/zeitgeist__zeitgeist,freedesktop-unofficial-mirror/zeitgeist__zeitgeist