compilers: Do not use -pthread with clang for darwin
compilers: Do not use -pthread with clang for darwin

Using the -pthread argument is not needed with clang when compiling for
darwin, and it results in the warning:

  warning: argument unused during compilation: '-pthread'

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