Makefile.in: fix copy of header files.

The makefile is apparently quite oudated, having some reference to some currently non-existing c/ecl/external.h file, which is now just h/external.h.

Copying all the header files was accidentally working thanks to the cp -rf $(srcdir)/h/*.h that existed, but it means that editing any header file (except the internal.h one) would not recompile appropriately.

This commit introduces an explicit way to declare the header files, and make can thus do its jobs correctly.