Fix make check build

make check on Ubuntu 13.04 with boost library 1.49 failed with
```
/bin/bash ./libtool --tag=CXX   --mode=link g++ -Icommon -Itools
-Ideps/libtap++-0.01
-DTESTDIR='"/home/vagrant/gearbox/plugins/status/t/sql"' -g -fpermissive
-o plugins/status/t/sql/status.t
plugins/status/t/sql/plugins_status_t_sql_status_t-status.t.o
deps/libtap++-0.01/libtap++.la gearbox/job/libgearbox_job.la
gearbox/core/libgearbox_core.la gearbox/store/libgearbox_store.la
libtool: link: g++ -Icommon -Itools -Ideps/libtap++-0.01
-DTESTDIR=\"/home/vagrant/gearbox/plugins/status/t/sql\" -g -fpermissive
-o plugins/status/t/sql/.libs/status.t
plugins/status/t/sql/plugins_status_t_sql_status_t-status.t.o
deps/libtap++-0.01/.libs/libtap++.so gearbox/job/.libs/libgearbox_job.so
-L/usr/lib -lgearman -lboost_graph_parallel
gearbox/core/.libs/libgearbox_core.so
gearbox/store/.libs/libgearbox_store.so -lsoci_core
/home/vagrant/gearbox/gearbox/core/.libs/libgearbox_core.so -lyajl
/usr/lib/x86_64-linux-gnu/libcurl.so -lboost_filesystem -llog4cxx
-lboost_regex-mt -lcrypto -luuid -lboost_iostreams-mt -Wl,-rpath
-Wl,/dev/null
/usr/bin/ld:
plugins/status/t/sql/plugins_status_t_sql_status_t-status.t.o: undefined
reference to symbol '_ZN5boost6system15system_categoryEv'
/usr/bin/ld: note: '_ZN5boost6system15system_categoryEv' is defined in
DSO /usr/lib/libboost_system.so.1.49.0 so try adding it to the linker
command line
/usr/lib/libboost_system.so.1.49.0: could not read symbols: Invalid
operation
collect2: error: ld returned 1 exit status
make[1]: *** [plugins/status/t/sql/status.t] Error 1
make[1]: Leaving directory `/home/vagrant/gearbox'
make: *** [check-am] Error 2
```
Added -lboost_system to fix the issue.