Improve compatibility with new CMake macro

Signed-off-by: Igor Raits <ignatenkobrain@fedoraproject.org>

kf5-kconfig.spec

@@ -137,12 +137,9 @@ PYTHONPATH=%{_datadir}/ECM/python
 export PYTHONPATH
 %endif
 
-mkdir %{_target_platform}
+%{cmake_kf5} -B %{_target_platform} \
-pushd %{_target_platform}
-%{cmake_kf5} .. \
   %{?ninja:-G Ninja} \
   %{?tests:-DBUILD_TESTING:BOOL=ON}
-popd
 
 %if 0%{?ninja}
 %ninja_build -C %{_target_platform}