Use get_default_include_dirs to detect BOOST_ROOT
Use get_default_include_dirs to detect BOOST_ROOT

This removes the need to specify BOOST_ROOT for cross-compilers.

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