[OMPI users] OMPI_C_DEF_PRE not set for MinGW build setup

[Roger Martin]

Hi,
On a Windows 7 64 bit box for mingw-w64 the CMake (version 2.8.6 and 
openmpi-1.5.5rc1) selection for MinGW generation doesn't put -D in front 
of preprocessor definitions such as _USRDLL, OMPI_IMPORTS, etc. 
OMPI_C_DEF_PRE=""