meson.build: enable CPP include check only in case CPP compiler is available

Drop hard meson C++/CPP dependency, only needed for the build-time
header inclusion test, build the test only in case C++/CPP compiler
is available.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>