Do not treat unused-const-variable warnings as errors
A code generation script in crda builds a .c file that is #include'd by
another .c file. There is a variable definition in the first .c file
that is used in the second .c file, but the compiler doesn't seem to
agree and generates an unused-const-variable warning. Since the default
CFLAGS treats warnings as errors, this warning needs special handling or
the build will fail...