cad/freecad: unbreak with boost 1.67

CMakeFiles/SMESH.dir/src/SMESH/SMESH_Mesh.cpp.o: In function `boost::condition_variable::condition_variable()':
src/3rdParty/salomesmesh/src/SMESH/SMESH_Mesh.cpp:(.text._ZN5boost18condition_variableC2Ev[_ZN5boost18condition_variableC2Ev]+0x33): undefined reference to `pthread_condattr_init'
src/3rdParty/salomesmesh/src/SMESH/SMESH_Mesh.cpp:(.text._ZN5boost18condition_variableC2Ev[_ZN5boost18condition_variableC2Ev]+0x4a): undefined reference to `pthread_condattr_setclock'
src/3rdParty/salomesmesh/src/SMESH/SMESH_Mesh.cpp:(.text._ZN5boost18condition_variableC2Ev[_ZN5boost18condition_variableC2Ev]+0x5f): undefined reference to `pthread_condattr_destroy'

PR: 227427
Reported by: antoine (via exp-run)