Create src/mkcustom.h which is included by config.h

Create src/mkcustom.h which is included by config.h

Put declarations for missing functions which we create in src/misc.c
into a file which is included by config.h via AH_BOTTOM().  This
ensures those prototypes are available, even in files added to
lib/... from gnulib.

* src/mkcustom.h: Add a new file with function declarations.
* configure.ac: Include src/mkcustom.h in config.h with AH_BOTTOM()
* Makefile.am: Add the header to the SRCS list.
* src/makeint.h: Remove content that we added to src/mkcustom.h.
* src/config.ami: Add #include "mkcustom.h" to specialized config.h.
* src/config.h-vms: Ditto.
* src/config.h.W32: Ditto.
* src/configh.dos: Ditto.