Fix igen build

Fix igen build

The igen build fails for me like:

gcc  -g -O2 -c ../../binutils-gdb/sim/igen/igen.c -o igen/igen.o
In file included from ../../binutils-gdb/sim/igen/igen.c:26:
../../binutils-gdb/sim/igen/lf.h:22:10: fatal error: ansidecl.h: No such file or directory

This patch fixes the problem by arranging for igen to find the
libiberty includes.

This seems slightly hacky to me, because libiberty is not a "build"
library, so it can't be linked against.  However, since igen currently
only includes the header, it seems relatively safe.

2021-05-04  Tom Tromey  <tromey@adacore.com>

	* Makefile.in: Rebuild.
	* Makefile.am (AM_CPPFLAGS): New variable.