Fix creation of nm.h when configure is changed

Fix creation of nm.h when configure is changed

My earlier patch -- commit c5adaa192 ("Fix creation of stamp-h by
gdb's configure script") -- broke the creation of nm.h.  In
particular, configure removes nm.h, so if you touch configure and
rebuild, nothing will re-create the link, breaking the build.

This patch fixes the bug, and also updates configure.ac to use
AC_CONFIG_LINKS, rather than the obsolete AC_LINK_FILES.

Finally, I noticed that gcore is in generated_files in the
Makefile.in.  I think this is incorrect, as generated_files is only
needed for files that can be the target of a #include.  So, this patch
removes it.

gdb/ChangeLog
2019-10-21  Tom Tromey  <tom@tromey.com>

	* configure.ac (nm.h): Conditionally create nm.h link.  Subst
	NM_H.   Use AC_CONFIG_LINKS.
	* configure: Rebuild.
	* Makefile.in (NM_H): New variable.
	(generated_files): Add NM_H.  Remove gcore.
	(nm.h, stamp-nmh): New targets.

Change-Id: I8dd539785d52455e85389425e4bb996c8a127a0e