compile pch .h file as c++-header when building a c++ target
compile pch .h file as c++-header when building a c++ target

Fixes #3641.

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