CMake: fix missing SOVERSION

[prj-]

Previous behavior:

$ otool -L petsc/arch-darwin-c-debug-real/lib/libparmmg.dylib
petsc/arch-darwin-c-debug-real/lib/libparmmg.dylib:
	petsc/arch-darwin-c-debug-real/lib/libparmmg.dylib (compatibility version 0.0.0, current version 0.0.0)
	petsc/arch-darwin-c-debug-real/lib/libmmg.5.dylib (compatibility version 5.0.0, current version 5.6.0)

New behavior:

$ otool -L petsc/arch-darwin-c-debug-real/lib/libparmmg.dylib
petsc/arch-darwin-c-debug-real/lib/libparmmg.dylib:
	petsc/arch-darwin-c-debug-real/lib/libparmmg.1.dylib (compatibility version 1.0.0, current version 1.4.0)
	petsc/arch-darwin-c-debug-real/lib/libmmg.5.dylib (compatibility version 5.0.0, current version 5.6.0)

This matches with the current code of Mmg.

[prj-]

@Algiane or @lcirrottola, could you please review/merge this? It's a very trivial change.

[Algiane]

Yes, of course (sorry, I missed the PR mail).
Thanks!

[prj-]

Thank you 😄