Fix #279 by not checking CMAKE_BUILD_TYPE when setting CMAKE_DEBUG_POSTFIX.

CMAKE_DEBUG_POSTFIX is already build-type specific and only applied
in case of "Debug" builds. The build-type is correctly inferred from
either CMAKE_BUILD_TYPE for MAkefile based generators or the selected
build type in the Visual Studio IDE. This fix allows to switch the
build type in Visual Studio and have Poco compile/link correctly.
Merge pull request #280 from saschazelzer/279-cmake-debug-postfix

Fix #279 by not checking CMAKE_BUILD_TYPE when setting CMAKE_DEBUG_POSTF...