cmake: find pthreads flags before OpenMP
cmake: find pthreads flags before OpenMP

CMake appears to have a bug where the pthreads compile flags will not be
detected if FindOpenMP is called, but then is OPENMP_FOUND is unset (as is
currently the case in Chrono).

rserban/chrono,PedroTrujilloV/chrono,dariomangoni/chrono,armanpazouki/chrono,Bryan-Peterson/chrono,armanpazouki/chrono,Milad-Rakhsha/chrono,dariomangoni/chrono,PedroTrujilloV/chrono,jcmadsen/chrono,rserban/chrono,tjolsen/chrono,andrewseidl/chrono,amelmquist/chrono,dariomangoni/chrono,andrewseidl/chrono,Milad-Rakhsha/chrono,amelmquist/chrono,hsu/chrono,amelmquist/chrono,hsu/chrono,andrewseidl/chrono,amelmquist/chrono,rserban/chrono,hsu/chrono,Milad-Rakhsha/chrono,Bryan-Peterson/chrono,tjolsen/chrono,amelmquist/chrono,projectchrono/chrono,jcmadsen/chrono,Bryan-Peterson/chrono,projectchrono/chrono,jcmadsen/chrono,armanpazouki/chrono,rserban/chrono,projectchrono/chrono,dariomangoni/chrono,armanpazouki/chrono,rserban/chrono,projectchrono/chrono,projectchrono/chrono,tjolsen/chrono,andrewseidl/chrono,rserban/chrono,hsu/chrono,rserban/chrono,tjolsen/chrono,Bryan-Peterson/chrono,amelmquist/chrono,jcmadsen/chrono,jcmadsen/chrono,projectchrono/chrono,armanpazouki/chrono,Milad-Rakhsha/chrono,Milad-Rakhsha/chrono,jcmadsen/chrono,Bryan-Peterson/chrono,Milad-Rakhsha/chrono,hsu/chrono,PedroTrujilloV/chrono,tjolsen/chrono,PedroTrujilloV/chrono,armanpazouki/chrono,dariomangoni/chrono,jcmadsen/chrono,PedroTrujilloV/chrono,dariomangoni/chrono,andrewseidl/chrono