correct path for CONFIGURE_FILE in CMakeLists.txt

If CMAKE_MODULE_PATH is empty before appending
'${CMAKE_CURRENT_SOURCE_DIR}/cmake' to it (CMakeLists.txt:27),
CONFIGURE_FILE (CMakeLists.txt:182) works as intended.
However, when CMAKE_MODULE_PATH is not empty, e.g., because of
further tool-chain settings, CONFIGURE_FILE fails with:

CMake Error: File <SOME-OTHER-PATH>;<POCO-SRC-PATH>/cmake/cmake_uninstall.cmake.in does not exist.
| CMake Error at CMakeLists.txt:182 (CONFIGURE_FILE):
|   configure_file Problem configuring file

This commit addresses this problem by only using
'${CMAKE_CURRENT_SOURCE_DIR}/cmake' in CONFIGURE_FILE.
The problem was discovered in the meta-ros layer project
(http://github.com/bmwcarit/meta-ros) when creating a libpoco
bitbake recipe, which uses poco's cmake installation scripts.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>