fix bug undefined reference to clock_gettime in Ubuntu 12.04.3

Ubuntu 12.04.3 
fix two compile error.

[ 86%] Building CXX object BitShares/sancho/CMakeFiles/sancho.dir/main.cpp.o
Linking CXX executable sancho
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_for(timespec const&)':
thread.cpp:(.text+0xf28): undefined reference to `clock_gettime'
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_until(timespec const&)':
thread.cpp:(.text+0x157d): undefined reference to `clock_gettime'
thread.cpp:(.text+0x15eb): undefined reference to `clock_gettime'
thread.cpp:(.text+0x1659): undefined reference to `clock_gettime'
thread.cpp:(.text+0x16c7): undefined reference to `clock_gettime'
/usr/local/lib/libboost_thread.a(thread.o):thread.cpp:(.text+0x1735): more undefined references to `clock_gettime' follow


Scanning dependencies of target bitshares_client
[ 87%] Building CXX object BitShares/server/CMakeFiles/bitshares_client.dir/messages.cpp.o
[ 87%] Building CXX object BitShares/server/CMakeFiles/bitshares_client.dir/client.cpp.o
/home/jeffrey/Git/keyhotee/BitShares/server/client.cpp: In member function ‘void client::connect_loop()’:
/home/jeffrey/Git/keyhotee/BitShares/server/client.cpp:165:21: warning: unused variable ‘connected’ [-Wunused-variable]
[ 88%] Building CXX object BitShares/server/CMakeFiles/bitshares_client.dir/bitshares_client_automoc.cpp.o
Linking CXX executable bitshares_client
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_for(timespec const&)':
thread.cpp:(.text+0xf28): undefined reference to `clock_gettime'
/usr/local/lib/libboost_thread.a(thread.o): In function `boost::this_thread::hiden::sleep_until(timespec const&)':
thread.cpp:(.text+0x157d): undefined reference to `clock_gettime'
thread.cpp:(.text+0x15eb): undefined reference to `clock_gettime'
thread.cpp:(.text+0x1659): undefined reference to `clock_gettime'
thread.cpp:(.text+0x16c7): undefined reference to `clock_gettime'
/usr/local/lib/libboost_thread.a(thread.o):thread.cpp:(.text+0x1735): more undefined references to `clock_gettime' follow