SO versions for cmake and libtool

this updates the version-sync script to generate proper SO versions
which will be used by cmake and libtool to create version symlinks
on the system when a library is installed as well as setting the SO
version in the binary.

To see what this does, you have to configure tox with a prefix:

    ./configure --prefix=/tmp/tox-with-libtool
    mkdir cbuild && cd cbuild && cmake -DCMAKE_INSTALL_PREFIX=/tmp/tox-with-cmake ..

Then run `make && make install`.

in both instances you should see the following installed in `lib/`:

    libtoxcore.so -> libtoxcore.so.1.4.0
    libtoxcore.so.1 -> libtoxcore.so.1.4.0
    libtoxcore.so.1.4.0

inside the binary the soname should be the one with .1 and it should not
contain the full version:

    $ objdump -p libtoxcore.so.1.4.0 | grep SONAME
      SONAME               libtoxcore.so.1

1 files changed, 2 insertions, 4 deletions

diff --git a/cmake/ModulePackage.cmake b/cmake/ModulePackage.cmake
index f6e7e31a..38f841ba 100644
--- a/cmake/ModulePackage.cmake
+++ b/cmake/ModulePackage.cmake
@@ -56,10 +56,8 @@ function(add_module lib)
     add_library(${lib}_shared SHARED ${ARGN})
     set_target_properties(${lib}_shared PROPERTIES
       OUTPUT_NAME ${lib}
-      VERSION ${PROJECT_VERSION}
-      # While on 0.x, the x behaves like the major version. 0.2 will be
-      # incompatible with 0.1. Change this, when releasing 1.0!
-      SOVERSION ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
+      VERSION ${SOVERSION}
+      SOVERSION ${SOVERSION_MAJOR}
     )
     install(TARGETS ${lib}_shared DESTINATION "lib")
   endif()