Boost timer config problem on some platforms

[Garth Wells]

The lines

# Boost timer was not a separate Boost component until version 1.48
if ("${Boost_MAJOR_VERSION}.${Boost_MINOR_VERSION}" VERSION_GREATER "1.47")
  find_package(Boost COMPONENTS timer REQUIRED)
endif()

in CMakeLists.txt cause a problem on some systems (Scientific Linux 6 + cmake 2.8.10.1). Calling find_package twice overwrites the Boost variables that were set when find_package(Boost) wad called before the above lines. This causes build failure.

To keep it simple, can we just require Boost > 1.47?

[Johannes Ring (johannr)]

If possible, I would like to keep support for older Boost versions for some more time. How about we instead do something like in the attached patch?