cmake: replaced custom debugExt variable with CMAKE_DEBUG_POSTFIX

CMakeLists.txt

examples/heat-equation/CMakeLists.txt

examples/inviscid-flow/CMakeLists.txt

examples/mean-curvature-flow/CMakeLists.txt

examples/narrow-band/CMakeLists.txt

examples/transport-equation/CMakeLists.txt

src/TNL/CMakeLists.txt

.../Hamilton-Jacobi/Solvers/fast-sweeping-map/CMakeLists.txt

...ntal/Hamilton-Jacobi/Solvers/fast-sweeping/CMakeLists.txt

...acobi/Solvers/hamilton-jacobi-parallel-map/CMakeLists.txt

...on-Jacobi/Solvers/hamilton-jacobi-parallel/CMakeLists.txt

...al/Hamilton-Jacobi/Solvers/hamilton-jacobi/CMakeLists.txt

src/Tools/CMakeLists.txt

src/UnitTests/CMakeLists.txt

src/UnitTests/Containers/CMakeLists.txt

src/UnitTests/Matrices/CMakeLists.txt

tests/benchmarks/CMakeLists.txt

tests/benchmarks/heat-equation-benchmark/CMakeLists.txt

tests/long-time-unit-tests/CMakeLists.txt

tests/mic/CMakeLists.txt