[XPTIFW][CMake] Set SONAME to the library major version (#23031)
xptifw sets only the library VERSION (1.0.1) and no SOVERSION, so the
generated SONAME is libxptifw.so.1.0.1. This churns the SONAME on every
point release, forcing needless rebuilds of consumers that link against
libxptifw, and it does not follow the usual shared-library convention.
Set SOVERSION to the project major version so the SONAME becomes
libxptifw.so.1, which stays stable across the 1.x series.
This is something I ran into while packaging v7.0.0 of the compiler for
Ubuntu 26.10. I have this change in a patch for the packaging but wanted
to go ahead and contribute it so ideally we don't need to maintain the
patch for future releases.
cc @dvrogozh @sarnex
