diagnose missing Python.h in configure

Compiling without python3-devel installed results in

./spot_impl_wrap.cxx:155:21: fatal error: Python.h: No such file or directory

it would be better to check for that in configure, and emit a more useful error message (suggest either to install python3-devel, or to use --disable-python).

Reported by Vitus Lam vitus.lam@hku.hk.