cmake: build boost debug variant when CMAKE_BUILD_TYPE is Debug

cmake: build boost debug variant when CMAKE_BUILD_TYPE is Debug

boost has some different predefined  build variants. they are quite
like CMake's CMAKE_BUILD_TYPE. in which, "debug" enables some
features related features. so it would be nice if we can have it
enabled for the Debug build, if the boost is built from source.
see also
https://www.boost.org/build/doc/html/bbv2/overview/builtins/features.html

before this change, we always build the "release" variant.  in this
change, "debug" variant is built if Ceph's is built with
CMAKE_BUILD_TYPE=Debug. please note, this change does not change
the way how boost is built when packaging Ceph, as our debian/rpm
receipts do not define CMAKE_BUILD_TYPE and respect the distros'
settings, in that case, the "release" variant is still built.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>