OpenMP in Release mode with the new compiler

I'm teaching myself OpenMP programming. The enclosed code (which I downloaded from the web) compiles and runs OK in Debug mode. In Release mode I get:

1>------ Build started: Project: OpenMP, Configuration: Release Win32 ------
1>Linking...
1>Simple.obj : error LNK2019: unresolved external symbol _omp_get_wtime referenced in function _MAIN__
1>Release\OpenMP.exe : fatal error LNK1120: 1 unresolved externals
1>
1>Build log written to  "file://c:\users\chris\documents\visual studio 2010\Projects\OpenMP\Release\BuildLog.htm"
1>OpenMP - 2 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

ChrisG