cmake: add a possibility to set library version

Add a new `ABIVERSION` define which allows to control the SOVERSION used
for the built shared library. This is needed for downstream packaging to
properly track breaking ABI changes when updating to newer versions of
the library.

Suggested-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Petr Štetiar <ynezz@true.cz>

CMakeLists.txt

@@ -92,3 +92,9 @@ IF(EXISTS ${json})
 	)
 
 ENDIF()
+
+IF(ABIVERSION)
+	SET_TARGET_PROPERTIES(ubox PROPERTIES VERSION ${ABIVERSION})
+	SET_TARGET_PROPERTIES(json_script PROPERTIES VERSION ${ABIVERSION})
+	SET_TARGET_PROPERTIES(blobmsg_json PROPERTIES VERSION ${ABIVERSION})
+ENDIF()