meson: set the soversion to just '0' everywhere

This is what glib did for the meson port, so can't be that wrong.
This also makes the DLL names match the autotools Windows DLLs.

gtk/meson.build

@@ -798,6 +798,7 @@ gtkversion_cdata.set('GTK_API_VERSION', gtk_api_version)
 gtkversion_cdata.set('GTK_BINARY_VERSION', gtk_binary_version)
 gtkversion_cdata.set('GTK_VERSION', meson.project_version())
 gtkversion_cdata.set('EXE_MANIFEST_ARCHITECTURE', '*')
+gtkversion_cdata.set('LT_CURRENT_MINUS_AGE', gtk_soversion)
 
 gtkversion = configure_file(input: 'gtkversion.h.in',
                             output: 'gtkversion.h',