backends: Don't hardcode system library paths
backends: Don't hardcode system library paths

Lookup the library paths using the available compilers instead.

This makes the code work on non-Linux platforms too.

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