zz40-xc-ovr.m4: provide XC_CONFIGURE_PREAMBLE macro
configure: use XC_CONFIGURE_PREAMBLE early checks

Some basic checks we make were placed early enough in generated
configure script when using autoconf 2.5X versions. Newer autoconf
versions expand these checks much further into the configure script,
rendering them useless. Using XC_CONFIGURE_PREAMBLE fixes placement
of early intended checks across all our autoconf supported versions.