guess_external_link_dependencies: deduplicate search dirs and libraries.
guess_external_link_dependencies: deduplicate search dirs and libraries.

Reduce speed impact of duplicated libs and pathes in the link command
line. (fixes #3465)

mesonbuild/meson,thiblahute/meson,jpakkane/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,MathieuDuponchelle/meson,becm/meson,pexip/meson,pexip/meson,pexip/meson,thiblahute/meson,MathieuDuponchelle/meson,jeandet/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,jeandet/meson,jpakkane/meson,mesonbuild/meson,jeandet/meson,pexip/meson,becm/meson,QuLogic/meson,thiblahute/meson,MathieuDuponchelle/meson,thiblahute/meson,jpakkane/meson,pexip/meson,pexip/meson,becm/meson,thiblahute/meson,thiblahute/meson,thiblahute/meson,jeandet/meson,QuLogic/meson,QuLogic/meson,becm/meson,becm/meson,becm/meson,thiblahute/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,jeandet/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,becm/meson,jpakkane/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,becm/meson,MathieuDuponchelle/meson,mesonbuild/meson,MathieuDuponchelle/meson,pexip/meson,becm/meson,thiblahute/meson,QuLogic/meson,jeandet/meson,pexip/meson,QuLogic/meson,jpakkane/meson,jeandet/meson,QuLogic/meson,jpakkane/meson,MathieuDuponchelle/meson,QuLogic/meson,jeandet/meson,jpakkane/meson