Remove code that checks for GNU/non-GNU make

Remove code that checks for GNU/non-GNU make

Since GNU make is now required to build GDB, we can remove everything
that checks whether the current make implemention is the GNU one or
not.  I simply removed the @GMAKE_TRUE@ prefixes and removed the whole
lines that were prefixed with @GMAKE_FALSE@.

I removed the code in the configure scripts that set those variables.

I also removed the following bits from the configure scripts:

  AC_CHECK_PROGS(MAKE, make): GNU make already defines a MAKE variable
    internally to be used when invoking Makefiles recursively.  I don't see
    this variable being used anywhere else (in scripts for example), so I
    think it's safe for removal.

  AC_PROG_MAKE_SET: This macro defines a SET_MAKE output variable, which
    is meant to be used in Makefiles to define the MAKE variable when
    using an implementation of make that doesn't already define it.
    Since we are now requiring GNU make, we don't need it anymore.
    Plus, I don't see SET_MAKE being used anywhere, so I don't think it
    was actually doing anything...

gdb/ChangeLog:

	* Makefile.in: Remove @GMAKE_TRUE@ prefixes and removes lines
	prefixed with @GMAKE_FALSE@.  Update comment related to non-GNU
	make.
	* configure.ac: Remove checks for the make program.
	* configure: Re-generate.

gdb/gdbserver/ChangeLog:

	* Makefile.in: Remove @GMAKE_TRUE@ prefixes and removes lines
	prefixed with @GMAKE_FALSE@.  Update comment related to non-GNU
	make.
	* configure.ac: Remove checks for the make program.
	* configure: Re-generate.

gdb/testsuite/ChangeLog:

	* Makefile.in: Remove @GMAKE_TRUE@ prefixes and removes lines
	prefixed with @GMAKE_FALSE@.  Update comment related to non-GNU
	make.
	* configure.ac: Remove checks for the make program.
	* configure: Re-generate.