configure: fix automake 1.13 compatibility

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
configure: autotools compatibility fixes - step I

Fix proper macro expansion order across autotools versions for
C compiler and preprocessor program checks.
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.
zz40-xc-ovr.m4: redirect errors and warnings to stderr
zz40-xc-ovr.m4: internals overhauling

- Update comments
- Execute commands in subshells
- Faster path separator check
- Fix missing 'test' command
- Rename private macros
- Minimize AC_REQUIRE usage
zz40-xc-ovr.m4: parentheses balancing of 'case' statements

m4 quadrigraph shell comment technique allows proper autoconf
parentheses balancing in shell 'case' statements. The presence
of unbalanced parentheses may otherwise trigger expansion bugs.
zz40-xc-ovr.m4: avoid double single-quote usage