detectenv_msvc.mak: Add MSVC 2008 x64-specific CFLAG

Due to an optimization bug in the Visual Studio 2008 x64 compiler/linker,
compilation of some programs shipped with g-i will hang, so use different
optimization settings for it, so that programs built through the NMake
Makefiles can build correctly.  A similar setting for the g-ir-compiler
is already in there.