Implement XDT_VERSION_INIT

Add an XDT_VERSION_INIT help macro that will define a set of version
macros using esyscmd so we no lnger need to use configure.ac.in to
perpare the version string for AC_INIT.

XDT_VERSION_INIT(SEMVER, [TAG] will set the following macros:
  - xdt_version
  - xdt_version_major
  - xdt_version_minor
  - xdt_version_micro
  - xdt_version_tag
  - xdt_version_build
  - xdt_debug_default

Example usage:

XDT_VERSION_INIT([4.15.3],[git])
AC_INIT([xfce4-someproject], [xdt_version()])
...
XDT_FEATURE_DEBUG([xdt_debug_default])