fix: build on windows

[laptou]

This library previously would not build on Windows. There are a few tricky parts that are not the fault of faiss-rs (ex.: you need to set MKL_ROOT env variable because CMake on Windows cannot locate Intel MKL automatically, and then it fails because no BLAS) but are maybe worth documenting. This PR is about fixing the library search path, though.

When I compile FAISS with CMake & MSVC on Windows, I get a build folder structure that looks like this:

C:\USERS\IBIYEMI\PROJECTS\NOT-MINE\FAISS\BUILD
│   ALL_BUILD.vcxproj
│   ALL_BUILD.vcxproj.filters
│   CMakeCache.txt
│   cmake_install.cmake
│   faiss.sln
│   INSTALL.vcxproj
│   INSTALL.vcxproj.filters
│   ZERO_CHECK.vcxproj
│   ZERO_CHECK.vcxproj.filters
│
├───c_api
│   │   ALL_BUILD.vcxproj
│   │   ALL_BUILD.vcxproj.filters
│   │   cmake_install.cmake
│   │   example_c.vcxproj
│   │   example_c.vcxproj.filters
│   │   faiss_c.vcxproj
│   │   faiss_c.vcxproj.filters
│   │   faiss_c_library.sln
│   │   INSTALL.vcxproj
│   │   INSTALL.vcxproj.filters
│   │
│   ├───CMakeFiles
│   │       generate.stamp
│   │       generate.stamp.depend
│   │
│   ├───faiss_c.dir
│   │   └───Release
│   │       │   AutoTune_c.obj
│   │       │   AuxIndexStructures_c.obj
│   │       │   clone_index_c.obj
│   │       │   (more object files, omitted for brevity)
│   │       │   MetaIndexes_c.obj
│   │       │   utils_c.obj
│   │       │   VectorTransform_c.obj
│   │       │
│   │       └───faiss_c.tlog
│   │               CL.command.1.tlog
│   │               Cl.items.tlog
│   │               CL.read.1.tlog
│   │               CL.write.1.tlog
│   │               CustomBuild.command.1.tlog
│   │               CustomBuild.read.1.tlog
│   │               CustomBuild.write.1.tlog
│   │               faiss_c.lastbuildstate
│   │               Lib-link.read.1.tlog
│   │               Lib-link.write.1.tlog
│   │               Lib.command.1.tlog
│   │
│   └───Release
│           faiss_c.lib
│
├───faiss
│   │   cmake_install.cmake
│   │   faiss.vcxproj
│   │   faiss.vcxproj.filters
│   │   faiss_avx2.vcxproj
│   │   faiss_avx2.vcxproj.filters
│   │   faiss_avx512.vcxproj
│   │   faiss_avx512.vcxproj.filters
│   │   faiss_avx512_spr.vcxproj
│   │   faiss_avx512_spr.vcxproj.filters
│   │   faiss_sve.vcxproj
│   │   faiss_sve.vcxproj.filters
│   │   INSTALL.vcxproj
│   │   INSTALL.vcxproj.filters
│   │
│   ├───CMakeFiles
│   │   │   generate.stamp
│   │   │   generate.stamp.depend
│   │   │
│   │   └───Export
│   │       └───5de5d61945e1ac7c8fc01cce1738f759
│   │               faiss-targets-debug.cmake
│   │               faiss-targets-minsizerel.cmake
│   │               faiss-targets-release.cmake
│   │               faiss-targets-relwithdebinfo.cmake
│   │               faiss-targets.cmake
│   │
│   ├───faiss.dir
│   │   └───Release
│   │       │   AdditiveQuantizer.obj
│   │       │   AutoTune.obj
│   │       │   AuxIndexStructures.obj
│   │       │   (more object files, omitted for brevity)
│   │       │   utils.obj
│   │       │   VectorTransform.obj
│   │       │   WorkerThread.obj
│   │       │
│   │       └───faiss.tlog (omitted for brevity)
│   │
│   └───Release
│           faiss.lib
│
└───x64 (this and other folders omitted for brevity)

But the build script was expecting faiss_c.lib to be directly inside of build\c_api, so the build fails.
I also got rid of the part that explicitly sets CMAKE_BUILD_TYPE to Release. The default behavior of the cmake crate is to do builds that follow the cargo build mode automatically, and setting the CMAKE_BUILD_TYPE causes the configuration and the build stages to use different profiles. This could lead to confusion.