mcompile: do not pass the builddir to ninja if it is the current directory
mcompile: do not pass the builddir to ninja if it is the current directory

Because when you don't specify -C, this logging feels silly:

ninja: Entering directory `.

mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,jpakkane/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,pexip/meson,mesonbuild/meson