Minor cmake fixes

[emmenlau]

This PR fixes a few minor issues for xtensor-blas:

• The variable xtensor_INCLUDE_DIR should probably be named xtensor_INCLUDE_DIRS?
• CMAKE_BUILD_TYPE should only be overwritten if not explicitly set by the user
• A duplicate call to find_package(xtensor REQUIRED) in https://github.com/xtensor-stack/xtensor-blas/compare/master...BioDataAnalysis:emmenlau_minor_cmake_fixes?expand=1#diff-15547c54d3d4898a882b4ab7b3cee381L113 leads to compile errors like:

  CMake Error at /data/Debug/lib/cmake/xtensor/xtensorConfig.cmake:53 (target_link_libraries):
  Cannot specify link libraries for target "xtensor" which is not built by this project.
  Call Stack (most recent call first):
  test/CMakeLists.txt:117 (find_package)

[JohanMabille]

Sorry for missing this one!

[emmenlau]

Awesome!