devel/libqb: Fix build and improve port

• Remove BUILD_DEPENDS hack, these will be a pain to fix if sprinkled all over the tree
• Remove Python dependency
• Add USES= pathfix
• Remove LDFLAGS variable (builds fine without)
• Fix build with DOXYGEN disabled
• Use gnome helper for libxml2
• Rearrage port for better readability

PR: 282570
Reviewed by: Älven <alster@vinterdalen.se> (maintainer)