Document new hard requirement on GNU make

Document new hard requirement on GNU make

As discussed in [1], it would be benificial for the GDB project to start
requiring GNU make to build its software.  It would allow using useful
GNU-specific constructs, such as pattern rules.  It would also allow
removing the alternative code paths in the Makefiles (guarded by
GMAKE_TRUE/GMAKE_FALSE), simplifying the Makefile code.

[1] https://sourceware.org/ml/gdb-patches/2016-11/msg00331.html

gdb/ChangeLog:

	* NEWS: Mention requirement of GNU make.