The machine configure scripts are all effectively stub scripts that
pass the higher level options to its own makefile. The only one doing
any custom tests was nds32. The rest were all effectively the same as
the libm/ configure script.
So instead of recursively running configure in all of these subdirs,
generate their makefiles from the top-level configure. For nds32,
deploy a pattern of including subdir logic via m4:
m4_include([machine/nds32/acinclude.m4])
Even its set of checks are very small -- it does 2 preprocessor tests
and sets up 2 makefile conditionals.
Some of the generated machine makefiles have a bunch of extra stuff
added to them, but that's because they were inconsistent in their
configure libtool calls. The top-level has it, so it exports some
new vars to the ones that weren't already.
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