Fix linking issue for Boost 1.50

[hobbes1069]

I'm not exactly sure why but if you don't include libboost_system linking fails with Boost 1.50. 

Here's the original error message prior to the "fix": inking CXX executable create_and_write /usr/bin/cmake -E cmake_link_script CMakeFiles/create_and_write.dir/link.txt --verbose=1 /usr/bin/c++   -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4  -m64 -mtune=generic -Wl,-z,relro  CMakeFiles/create_and_write.dir/apps/sample_code/create_and_write/main.cpp.o  -o create_and_write -rdynamic libField3D.so.1.3 -lhdf5 -lz -lrt -lm -lz -lIex -lHalf -lIlmThread -lImath -ldl -lboost_thread-mt -lboost_program_options-mt -Wl,-rpath,/builddir/build/BUILD/imageworks-Field3D-94d1bf3/build /usr/bin/ld: CMakeFiles/create_and_write.dir/apps/sample_code/create_and_write/main.cpp.o: undefined reference to symbol '_ZN5boost6system15system_categoryEv' /usr/bin/ld: note: '_ZN5boost6system15system_categoryEv' is defined in DSO /lib64/libboost_system-mt.so.1.50.0 so try adding it to the linker command line /lib64/libboost_system-mt.so.1.50.0: could not read symbols: Invalid operation collect2: error: ld returned 1 exit status

[derek-gerstmann]

Confirm exact same issue building Field3D trunk on OSX 10.8 using Boost 1.50.

[hobbes1069]

Looks like we definitly need libboost_system.

https://svn.boost.org/trac/boost/ticket/7241