Fix cross file initialization of CPPFLAGS
Fix cross file initialization of CPPFLAGS

'c_arg' entries should become *both* compiler options and external
peprocessor options for C. (And likewise for a few other languages.)
Seems inconsistent to me, but this is the status quo.

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