Tested with:
buildconf: autoconf version 2.69
buildconf: autom4te version 2.69
buildconf: autoheader version 2.69
buildconf: automake version 1.13.1
buildconf: aclocal version 1.13.1
buildconf: libtool version 2.4
buildconf: GNU m4 version 1.4.16
Provide a 'traceable' AC_CONFIG_MACRO_DIR definition only when using
an autoconf version that does not provide it, instead of what we were
doing up to now of providing and overriding AC_CONFIG_MACRO_DIR for
all autoconf versions.