lib: use libtool like versioning

In order to be safe for future ABI changes and to fulfill packaging
requirements for e.g. Fedora, provide libtool like versioning for
both libraries, i.e. libgamemode and libgamemodeauto.

lib/meson.build

@@ -1,3 +1,10 @@
+# Libtool like versioning (current.revision.age) for the libraries
+# See https://www.sourceware.org/autobook/autobook/autobook_61.html#Library-Versioning
+lt_current = '0'
+lt_revision = '0'
+lt_age = '0'
+lt_version = '@0@.@1@.@2@'.format(lt_current, lt_age, lt_revision)
+
 # Main client library to message the daemon
 gamemode = shared_library(
     'gamemode',
@@ -8,6 +15,8 @@ gamemode = shared_library(
         dep_systemd,
     ],
     install: true,
+    soversion: lt_current,
+    version: lt_version,
 )
 
 libgamemode_includes = [
@@ -24,6 +33,8 @@ gamemodeauto = shared_library(
         libdl,
     ],
     install: true,
+    soversion: lt_current,
+    version: lt_version,
 )
 
 # Install the gamemode_client header