check that GNU is defined before comparing

svn path=/trunk/boost/boost/date_time/; revision=16642
Update V2 Jamfile for Boost.

svn path=/trunk/boost/libs/date_time/; revision=16882
Change use-requirements to usage-requirements in some library Jamfiles.

svn path=/trunk/boost/libs/date_time/; revision=16884
update return value

svn path=/trunk/boost/libs/date_time/; revision=16938
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16939
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16940
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16941
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16942
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16943
fix return value

svn path=/trunk/boost/libs/date_time/; revision=16944
new testing jamfile

svn path=/trunk/boost/libs/date_time/; revision=16945
remove commented tests

svn path=/trunk/boost/libs/date_time/; revision=16947
add Jamfile for examples

svn path=/trunk/boost/libs/date_time/; revision=16949
Fix problem with DATE_TIME_INLINE macro

svn path=/trunk/boost/boost/date_time/; revision=16950
remove DATE_TIME_INLINE macro

svn path=/trunk/boost/libs/date_time/; revision=16951
modernize Jamfile, remove DATE_TIME_INLINE bug

svn path=/trunk/boost/libs/date_time/; revision=16952
remove DATE_TIME_INLINE macro

svn path=/trunk/boost/libs/date_time/; revision=16953
add info on julian and week number interfaces

svn path=/trunk/boost/libs/date_time/; revision=16954
fix error in code

svn path=/trunk/boost/libs/date_time/; revision=16955
add the missing year_iterator typedef

svn path=/trunk/boost/boost/date_time/; revision=16956