Revert previous change to tools/Makefile.gnuhost. When getenv("M4")
contains multiple words, flex fails.

Adjust tools/autoconf/Makefile to pass the necessary extra arguments via
M4=... in CONFIGURE

Revert previous change to tools/Makefile.gnuhost. When getenv("M4")
contains multiple words, flex fails.

Adjust tools/autoconf/Makefile to pass the necessary extra arguments via
M4=... in CONFIGURE_ENV. This is similar to what used to be in
revision 1.4 of tools/autoconf/Makefile, but it's now after instead of
before the inclusion of Makefile.gnuhost, because it has to override the
value of M4=... that Makefile.gnuhost stores in CONFIGURE_ENV.

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Add the "-g -D__gnu__" flags when invoking M4 via CONFIGURE_ARGS
in tools/Makefile.gnuhost. This makes the tools version of m4
behave like GNU m4.

Remove similar code from tools/autoconf/Makefile,

Add the "-g -D__gnu__" flags when invoking M4 via CONFIGURE_ARGS
in tools/Makefile.gnuhost. This makes the tools version of m4
behave like GNU m4.

Remove similar code from tools/autoconf/Makefile, where it didn't
work because Makefile.gnuhost would previously have overridden it.

This fixes a problem in which the tools version of autoconf complained
"M4sugar requires GNU M4.
Install it before installing M4sugar or set the M4 environment variable to its path name."

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Centralize the definition of the tool name prefix (i.e. "nb") into
bsd.own.mk, as the _TOOL_PREFIX variable.

Use ${TOOLDIR}/bin/nbm4.

Prefix all target-specific things with `${MACHINE_GNU_PLATFORM}-'.
Prefix all non-target-specific things with `nb'.

Add autoconf as a host tool, not built by default.