Compare scan-build's [documentation][1]:
> ALWAYS analyze a project in its "debug" configuration
>
> Most projects can be built in a "debug" mode that enables assertions.
> Assertions are picked up by the static analyzer to prune infeasible
> paths, which in some cases can greatly reduce the number of false
> positives (bogus error reports) emitted by the tool.
Closes#3960.
1: http://clang-analyzer.llvm.org/scan-build.html#recommended_debug
This PR enables the use of clang++ to compile C++ projects by adding
`CXXUWFLAGS += -fno-delete-null-pointer-checks`, if `${CC}` understands
`-fno-delete-null-pointer-checks` but `${CXX}` does not.
