On OS X 10.7 building against boost trunk requires linking of all plugins to boost_system

[springmeyer]

This was reported by one user (@epifanio) and now I've upgraded to boost trunk and hit it. It seems possible it is a config error of some sort, but there also does not seem to be much harm with linking boost_system for all plugins, since those that use filesystem already do.

$ /Developer/usr/bin/clang++ -o plugins/input/gdal.input -dynamiclib plugins/input/gdal/gdal_datasource.os 
plugins/input/gdal/gdal_featureset.os -Ldeps/agg -Lsrc -L/opt/boost-trunk/lib -L/usr/local/Cellar/postgresql/9.1.2/lib -L/usr/local/
Cellar/geos/3.3.2/lib -L/usr/local/Cellar/gdal/1.9.0/lib -L/usr/local/Cellar/icu4c/4.8.1.1/lib -L/usr/local/lib -L/usr/lib -L/usr/X1
1/lib -lgdal -lmapnik -licuuc
Undefined symbols for architecture x86_64:
  "boost::system::generic_category()", referenced from:
      __GLOBAL__I_a in gdal_datasource.os
      __GLOBAL__I_a in gdal_featureset.os
  "boost::system::system_category()", referenced from:
      __GLOBAL__I_a in gdal_datasource.os
      __GLOBAL__I_a in gdal_featureset.os
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
~/projects/mapnik[master]$ g++ -o plugins/input/gdal.input -dynamiclib plugins/input/gdal/gdal_datasource.os plugins/input/gdal/gdal
_featureset.os -Ldeps/agg -Lsrc -L/opt/boost-trunk/lib -L/usr/local/Cellar/postgresql/9.1.2/lib -L/usr/local/Cellar/geos/3.3.2/lib -
L/usr/local/Cellar/gdal/1.9.0/lib -L/usr/local/Cellar/icu4c/4.8.1.1/lib -L/usr/local/lib -L/usr/lib -L/usr/X11/lib -lgdal -lmapnik -
licuuc
Undefined symbols for architecture x86_64:
  "boost::system::generic_category()", referenced from:
      global constructors keyed to ain gdal_datasource.os
      global constructors keyed to ain gdal_featureset.os
  "boost::system::system_category()", referenced from:
      global constructors keyed to ain gdal_datasource.os
      global constructors keyed to ain gdal_featureset.os
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status