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, 99 insertions, 1 deletions

diff --git a/other/version-sync b/other/version-sync
index 3fb1aa80..fdfe91d9 100755
--- a/other/version-sync
+++ b/other/version-sync
@@ -17,7 +17,9 @@ update() {
   if diff "$file" "$file.updated-version"; then
     rm "$file.updated-version"
   else
-    mv "$file.updated-version" "$file"
+    # use cat > and rm instead of move to keep file permissions
+    cat "$file.updated-version" > "$file"
+    rm "$file.updated-version"
   fi
 }
 
@@ -26,3 +28,99 @@ update 'configure.ac' 's/AC_INIT(\[tox\], \[.*\])/AC_INIT([tox], ['$VER'])/'
 update 'toxcore/tox.api.h' 's/\(const VERSION_MAJOR *= \).*;/\1'$MAJOR';/'
 update 'toxcore/tox.api.h' 's/\(const VERSION_MINOR *= \).*;/\1'$MINOR';/'
 update 'toxcore/tox.api.h' 's/\(const VERSION_PATCH *= \).*;/\1'$PATCH';/'
+
+#
+# calculating the SO version
+#
+# The SO version reflects changes in the ABI compatibility of the libary [1].
+# The general convention on this is that the SO version is a monotonically
+# increasing number. The major version reflects breaking changes and the minor
+# number reflect non-breaking updates [2].
+#
+# The SO version for tox libraries consists of two parts: `A.B`.
+# - incrementing A reflects an ABI breaking change.
+# - incrementing B reflects a non-ABI-breaking update. B is set to 0 when A is incremented.
+#
+# As the tox versioning scheme directly reflects ABI compatibility, we can use it
+# to construct the SO version.
+#
+# In the `0.y.z` release cycle, breaking changes are allowed in every increment of `y`,
+# so we can build the SO version just by taking `y.z`.
+# In the `x.y.z` release cycle with `x > 0` the SO version must be calculated by taking
+# the major of the tox version and add a hardcoded number that is the last A of the 0.y.z
+# release cycle.
+#
+# References:
+#
+# [1]: https://autotools.io/libtool/version.html
+# [2]: http://www.ibm.com/developerworks/linux/library/l-shlibs/index.html#N1006E
+#
+
+# the last major version number from the 0.x release cycle
+# this must be constant starting from the 1.0 release
+LAST_SOMAJOR=1
+
+if [ $MAJOR -eq 0 ]; then
+    SOMAJOR=$MINOR
+    SOMINOR=$PATCH
+
+    # update lastmajor above
+    update 'other/version-sync' 's/^\(LAST_SOMAJOR=\).*/\1'$SOMAJOR'/'
+else
+    SOMAJOR=$(expr $MAJOR + $LAST_SOMAJOR)
+    SOMINOR=$MINOR
+fi
+
+#
+# libtool has a quite cryptic implementation of the versioning system, which also
+# changes between systems, see https://github.com/lxde/lxqt/issues/488#issuecomment-238084222
+#
+# .so library version, following the libtool scheme:
+#
+#            current:revision:age
+#
+# current:  increment if interfaces have been added, removed or changed
+# revision: increment if source code has changed, set to zero if current is
+#           incremented
+# age:      increment if interfaces have been added, set to zero if
+#           interfaces have been removed or changed
+#
+# For a full reference see:
+# https://www.gnu.org/software/libtool/manual/libtool.html#Updating-version-info
+#
+# Passing such a version string to libtool will generate the following version number
+# on the libary binary file on GNU/Linux:
+#
+# (current - age).(age).(revision)
+#
+# We do not want to use a separate version numbering for the library because the package versioning is equally good:
+#
+# Semver non-breaking: 0.y.(z+1) or x.(y+1).0:
+#
+# This would mean to increment current and age, which leave the major of SOVERSION the same.
+# Revision is incremented.
+#
+# Semver breaking: 0.(y+1).0 or (x+1).0.0:
+#
+# This would mean to increment current, set age and revision to zero.
+#
+# Thus to make libtool use our version, we have to pass (major + minor):patch:minor as current:revision:age to get:
+#
+#     (current - age).(age).(revision)
+# <=> (major + minor - minor).minor.patch
+# <=> major.minor.patch
+#
+
+if [ $MAJOR -eq 0 ]; then
+    LIBTOOL_CURRENT=$(expr $SOMAJOR + $SOMINOR)
+    LIBTOOL_AGE=$SOMINOR
+    LIBTOOL_REVISION=0
+else
+    LIBTOOL_CURRENT=$(expr $SOMAJOR + $SOMINOR)
+    LIBTOOL_AGE=$SOMINOR
+    LIBTOOL_REVISION=$PATCH
+fi
+
+update 'so.version' 's/^\(CURRENT=\).*/\1'$LIBTOOL_CURRENT'/'
+update 'so.version' 's/^\(AGE=\).*/\1'$LIBTOOL_AGE'/'
+update 'so.version' 's/^\(REVISION=\).*/\1'$LIBTOOL_REVISION'/'