Fix vstudio build

Dear colleagues, please consider these 3 simple commits, fixing OpenMVG build in Visual Studio 2013, with cmake 3.0.1.
As for CMAKE_BUILD_TYPE, I propose to leave this variable unset for VS, since ["this variable is only meaningful to single-configuration generators (such as make and Ninja)"](http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_BUILD_TYPE.html)