Rather than hand maintain m4 includes in various autotool files,
use AC_CONFIG_MACRO_DIRS to declare the relevant search paths.
This simplifies the code, makes it more robust, and cleans out
unused logic from configure.
The AC_CONFIG_HEADER macro is long deprecated, so switch to the
newer form. This also gets rid of the position limitation, and
drops support for an argument to SIM_AC_COMMON which we haven't
used anywhere.
These settings might have made sense in darker compiler times, but I
think they're largely obsolete now. Looking through the values that
get used in HDEFINES, it's quite limited, and configure itself should
handle them. If we still need something, we can leverage standard
autoconf macros instead, after we get a clear user report.
TDEFINES was never set anywhere and was always empty, so prune that.
This is a hand-written implementation that should have fairly complete
coverage for the base integer instruction set ("i"), and for the atomic
("a") and integer multiplication+division ("m") extensions. It also
covers 32-bit & 64-bit targets.
The unittest coverage is a bit weak atm, but should get better.
