cmake: build executables, libraries and bindings files on `obj` dir

After building SPTAG and then running make clean and deleting the obj directory, the files generated by the build (including swig bindings files) are still present.

None
None/sptag-indexbuilder
None/sptag-server
None/sptag-client
None/_SPTAG.so
None/SPTAG.py
None/test
None/sptag-aggregator
None/SPTAGClient.py
None/libSPTAGLib.so
None/_SPTAGClient.so
None/libSPTAGLibStatic.a
None/sptag-indexsearcher
None/libDistanceUtils.a
Wrappers/inc/ClientInterface_pwrap.cpp
Wrappers/inc/CoreInterface_pwrap.cpp
Wrappers/inc/SPTAG.py
Wrappers/inc/SPTAGClient.py

The .gitignore file already ignores some of these files:

/Wrappers/inc/*wrap.*
/Wrappers/inc/*.py
/Wrappers/inc/*.java

Since these files are generated during the build, they should be output to the build dir (usually obj).

I noticed that on Windows, the CMakeLists.txt

$ grep -C4 CMAKE_BINARY_DIR CMakeLists.txt
if (WIN32)
    set(Boost_USE_STATIC_LIBS ON)

    set(CMAKE_CONFIGURATION_TYPES ${CMAKE_BUILD_TYPE})
    set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
    set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})
    set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})

    set (LIBRARY_OUTPUT_PATH ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR})
    set (EXECUTABLE_OUTPUT_PATH ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR})
else()
    set (LIBRARY_OUTPUT_PATH "${PROJECT_SOURCE_DIR}/${CMAKE_BUILD_TYPE}/")
    set (EXECUTABLE_OUTPUT_PATH "${PROJECT_SOURCE_DIR}/${CMAKE_BUILD_TYPE}/")
endif()

I suggest that the output directories be set to ${CMAKE_BINARY_DIR} and that the swig bindings be built in ${CMAKE_BINARY_DIR} too.

Start with CMakeLists.txt and trace the output-directory settings for executables, libraries, archives, and SWIG-generated bindings. Reproduce the build, make clean, and obj-directory removal, then verify that generated artifacts are placed under the build directory and are removed without leaving files in the source tree.