[cmake] Fix shared library versioning on OpenBSD.

This emulates Libtool to make sure Libtool and CMake agree on the ABI
version. Specifically, for OpenBSD, `builds/unix/configure` defines
`version_type=sunos` and skips defining `soname_spec`.

See upstream MR: https://gitlab.freedesktop.org/freetype/freetype/-/merge_requests/433.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -458,9 +458,17 @@ if (WIN32)
 endif ()
 
 if (BUILD_SHARED_LIBS)
-  set_target_properties(freetype PROPERTIES
-    VERSION ${LIBRARY_VERSION}
-    SOVERSION ${LIBRARY_SOVERSION})
+  if (CMAKE_SYSTEM_NAME STREQUAL "OpenBSD")
+    # For OpenBSD, libtool follows `sunos` version type.
+    set_target_properties(freetype PROPERTIES
+      # libtool does not define its internal `soname_spec` variable
+      # for OpenBSD. So skipping the SOVERSION option.
+      VERSION ${LIBTOOL_CURRENT}.${LIBTOOL_REVISION})
+  else ()
+    set_target_properties(freetype PROPERTIES
+      VERSION ${LIBRARY_VERSION}
+      SOVERSION ${LIBRARY_SOVERSION})
+  endif ()
 endif ()
 
 # Pick up ftconfig.h and ftoption.h generated above, first.