qt, cpp: Fix versioning in cmake config and header

* configure.ac (VERSION_MAJOR, VERSION_MINOR, VERSION_MICRO): New
subst variables for the version header.
* lang/cpp/src/GpgmeppConfigVersion.cmake.in,
lang/cpp/src/gpgmepp_version.h.in,
lang/qt/src/QGpgmeConfigVersion.cmake.in,
lang/qt/src/qgpgme_version.h.in: Use new variables.

Using the LT_* variables was just wrong. Reporting the
package version also makes more sense then the library version.
Having different versions might make ABI breaks more visible
by increasing the major version number, but to have different
versions in the same package is too confusing imo and gpgme
uses a versioning that is unrelated to the library version number.