meson: Preserve compatibility with Autotools build

The Meson build is missing the libtool versioning of the shared
libraries.

gdk/meson.build

@@ -280,6 +280,7 @@ endif
 
 libgdk = shared_library('gdk-3',
                         soversion: gtk_soversion,
+                        version: gtk_library_version,
                         sources: [gdk_sources, gdk_backends_gen_headers, gdkconfig],
                         dependencies: gdk_deps,
                         include_directories: [confinc, gdkx11_inc, wlinc],

gtk/meson.build

@@ -966,6 +966,7 @@ endforeach
 # Library
 libgtk = shared_library('gtk-3',
                         soversion: gtk_soversion,
+                        version: gtk_library_version,
                         sources: [typefuncs, gtk_sources, gtkmarshal_h, gtkprivatetypebuiltins_h],
                         c_args: gtk_cargs + common_cflags,
                         include_directories: [confinc, gdkinc, gtkinc],

meson.build

@@ -108,6 +108,10 @@ else
   gtk_soversion = '0'
 endif
 
+# Maintain compatibility with the Autotools build
+current = gtk_binary_age - gtk_interface_age
+gtk_library_version = '@0@.@1@.@2@'.format(gtk_soversion, current, gtk_interface_age)
+
 gtk_osxversions = [(100 * gtk_minor_version) + 1, '@0@.@1@.0'.format((100 * gtk_minor_version) + 1, gtk_micro_version)]
 
 gtk_api_version = '@0@.0'.format(gtk_major_version)