Doxygen: Fix an issue causing missing #define documentation output.
Searching for documented defines...
[...]libserialport.h:1624: warning: documentation for unknown define SP_PACKAGE_VERSION_MAJOR found.
[...]libserialport.h:1627: warning: documentation for unknown define SP_PACKAGE_VERSION_MINOR found.
[...]libserialport.h:1630: warning: documentation for unknown define SP_PACKAGE_VERSION_MICRO found.
[...]libserialport.h:1633: warning: documentation for unknown define SP_PACKAGE_VERSION_STRING found.
[...]libserialport.h:1640: warning: documentation for unknown define SP_LIB_VERSION_CURRENT found.
[...]libserialport.h:1643: warning: documentation for unknown define SP_LIB_VERSION_REVISION found.
[...]libserialport.h:1646: warning: documentation for unknown define SP_LIB_VERSION_AGE found.
[...]libserialport.h:1649: warning: documentation for unknown define SP_LIB_VERSION_STRING found.