CMake builds: avoid NDEBUG in RelWithDebInfo builds

Just like is done for Release builds, NDEBUG shouldn't be set for RelWithDebInfo
builds either. Although we are trying to move away from straight "assert" and
use invariants instead, we a) haven't completed this yet and b) imported code
such as optional.hpp continues to use "assert."

Author: tautschnig

Diff for: CMakeLists.txt

@@ -22,6 +22,7 @@ if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang" OR
 )
     #   Ensure NDEBUG is not set for release builds
     set(CMAKE_CXX_FLAGS_RELEASE "-O2")
+    set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -g")
     #   Enable lots of warnings
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wpedantic -Werror -Wno-deprecated-declarations")
 elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")