Fix race condition when building with OCE_USE_PCH=ON OCE_COPY_HEADERS_BUILD=ON
Fix race condition when building with OCE_USE_PCH=ON OCE_COPY_HEADERS_BUILD=ON

See for instance travis build failure for job 257.5.

heartvalve/oce,Tridify/oce,heartvalve/oce,EvgeneOskin/oce,Tech-XCorp/oce,tpaviot/oce,Tridify/oce,Tech-XCorp/oce,heartvalve/oce,mathstuf/oce,tpaviot/oce,BenoitPerrot/oce,EvgeneOskin/oce,EvgeneOskin/oce,finetjul/oce,finetjul/oce,EvgeneOskin/oce,tpaviot/oce,mathstuf/oce,tpaviot/oce,Tech-XCorp/oce,Tridify/oce,BenoitPerrot/oce,BenoitPerrot/oce,Tech-XCorp/oce,finetjul/oce,BenoitPerrot/oce,heartvalve/oce,BenoitPerrot/oce,finetjul/oce,tpaviot/oce,mathstuf/oce,heartvalve/oce,EvgeneOskin/oce,Tridify/oce,finetjul/oce,Tech-XCorp/oce,mathstuf/oce,mathstuf/oce,Tridify/oce