build: fix automake serial-tests check again

This effectively undoes the following commits:

    a97685e build: add automake serial-tests version check
    e4c1483 build: serial-tests was added in automake v1.12

After much trial and error I've come to the conclusion that you cannot
reliably test for the automake version inside configure.ac itself.

Feature checks (testing for the presence of macros with m4_ifdef) is
not reliable when the macro is "lazy-loaded" by aclocal: m4 won't see
its definition unless it's actually used in configure.ac, hence checking
for obsolete macros like AM_ENABLE_MULTILIB and AM_WITH_REGEX is not a
reliable proxy for the automake version - both are are lazy-loaded.

That's why this commit moves the version check to autogen.sh, creates
a m4 file with automake options on the fly and includes that in
configure.ac.

Thank you, automake maintainers, for making hard what should be easy.
That's an hour of my life I won't be getting back!