p3dfft++ and p3dfft C header files have the same name

[luszczek]

p3dfft++ has p3dfft.3/include/p3dfft.h and p3dfft has p3dfft/include/p3dfft.h. The former is the declaration file for the implementation that's safe for use by a C compiler. The latter it is a set of wrapper that needs config.h to wrap Fortran implementation based on the compiler used. This means that a file with a line:

#include <p3dfft.h>

could be using either library depending on the environment settings and compiler flags.