cmake: Fix conflicting optimization flags in debug builds

[a-szegel]

• Change CONFIG:Debug to CONFIG:debug for case sensitivity
• Remove trailing semicolons in flag lists
• Replace $<IF:...> with mutually exclusive $<$<condition>:...> for CUDA flags to avoid false branch evaluation on C++ files

Fixes debug builds incorrectly getting both -O0 and -O3 flags.

On Devel:

> cd /home/szegel/Nvshmem && rm -rf build && mkdir build && cd build && cmake .. -DNVSHMEM_BUILD_PYTHON_LIB=0 -DNVSHMEM_DEBUG=ON -DNVSHMEM_DEVEL=ON > /dev/null 2>&1 && grep "CXX_FLAGS" /home/szegel/Nvshmem/build/src/CMakeFiles/nvshmem_transport_libfabric.dir/flags.make
CXX_FLAGS = -g -std=gnu++11 -fPIC -O0 -g -O3 -Werror -Wall -Wextra -Wno-unused-function -Wno-unused-parameter -Wno-missing-field-initializers


>  cd /home/szegel/Nvshmem && rm -rf build && mkdir build && cd build && cmake .. -DNVSHMEM_BUILD_PYTHON_LIB=0 -DNVSHMEM_DEVEL=ON > /dev/null 2>&1 && grep "CXX_FLAGS" /home/szegel/Nvshmem/build/src/CMakeFiles/nvshmem_transport_libfabric.dir/flags.make
CXX_FLAGS = -O3 -DNDEBUG -std=gnu++11 -fPIC -O3 -Werror -Wall -Wextra -Wno-unused-function -Wno-unused-parameter -Wno-missing-field-initializers

With this PR (no -O0 -g -O3 in debug build):

>  cd /home/szegel/Nvshmem && rm -rf build && mkdir build && cd build && cmake .. -DNVSHMEM_BUILD_PYTHON_LIB=0 -DNVSHMEM_DEBUG=ON -DNVSHMEM_DEVEL=ON > /dev/null 2>&1 && grep "CXX_FLAGS" /home/szegel/Nvshmem/build/src/CMakeFiles/nvshmem_transport_libfabric.dir/flags.make
CXX_FLAGS = -g -std=gnu++11 -fPIC -O0 -g -Werror -Wall -Wextra -Wno-unused-function -Wno-unused-parameter -Wno-missing-field-initializers


> cd /home/szegel/Nvshmem && rm -rf build && mkdir build && cd build && cmake .. -DNVSHMEM_BUILD_PYTHON_LIB=0 -DNVSHMEM_DEBUG=OFF -DNVSHMEM_DEVEL=ON > /dev/null 2>&1 && grep "CXX_FLAGS" /home/szegel/Nvshmem/build/src/CMakeFiles/nvshmem_transport_libfabric.dir/flags.make
CXX_FLAGS = -O3 -DNDEBUG -std=gnu++11 -fPIC -O3 -Werror -Wall -Wextra -Wno-unused-function -Wno-unused-parameter -Wno-missing-field-initializers