This removes the -DNDEBUG flag from the CFLAGS and CXXFLAGS even when
this flag is set as part of a selected build type.

For instance the RelWithDebInfo build type CXXFLAGS default to `-g -O2
-DNDEBUG`. This diffs removes the NDEBUG leading to -g -O2 being set
instead.

It completes the remove_compiler_flag macro to achieve this, and fixes
some edge cases where the macro would have fail:

• if the flag to remove is located at the end (no trailing space)
• if any of the *_FLAGS variable is empty or unset

mkdir buildcmake && cd buildcmake
cmake -GNinja .. -DCMAKE_BUILD_TYPE=RelWithDebInfo
ninja -v

Check that the -g -O2 flags are set in the compile commands for both
CC and CXX compile lines. Ensure that the -DNDEBUG flag is never set.