RF: write config.h rather than config.pxi file
RF: write config.h rather than config.pxi file

Signal that code should use C-level conditionals rather than
Cython-level conditionals by writing .h file instead of .pxi.

mdesco/dipy,demianw/dipy,sinkpoint/dipy,nilgoyyou/dipy,matthieudumont/dipy,jyeatman/dipy,Messaoud-Boudjada/dipy,nilgoyyou/dipy,villalonreina/dipy,matthieudumont/dipy,JohnGriffiths/dipy,rfdougherty/dipy,mdesco/dipy,villalonreina/dipy,rfdougherty/dipy,StongeEtienne/dipy,oesteban/dipy,FrancoisRheaultUS/dipy,Messaoud-Boudjada/dipy,sinkpoint/dipy,StongeEtienne/dipy,oesteban/dipy,demianw/dipy,FrancoisRheaultUS/dipy,JohnGriffiths/dipy,beni55/dipy,beni55/dipy,jyeatman/dipy