cmakelist: fix msvc defs leaking to other platform
cmakelist: fix msvc defs leaking to other platform

The MSVC_VERSION check was kicking in on Linux (and presumably OSX),
and causing redefines of `inline` and `snprintf`, and failing the
build with  lots of "undefined reference to `_snprintf'" errors.

bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr