Bring back dmpstruct.h

Bring back dmpstruct.h

Bring back the dmpstruct.h checking, and use it when
--enable-checking=structs is specified.  The checking can be helpful
to some developers, although it gets in the way of others and is
not needed for ordinary tarball builds.
* src/dmpstruct.awk: Restore this file, with mode 644 not 755.
* configure.ac: New option-arg --enable-checking=structs,
implied by --enable-checking.
(CHECK_STRUCTS): New macro and var.
* src/Makefile.in (CHECK_STRUCTS): New macro.
(dmpstruct_headers, dmpstruct.h, dmpstruct.h):
Restore these macros and rules.
(pdumper.o): Restore this dependency if $(CHECK_STRUCTS) is true.
(mostlyclean): Remove dmpstruct.h.
* src/pdumper.c [CHECK_STRUCTS]: Include dmpstruct.h,
and restore checks against hashes.