configure: document that -Wwrite-strings is missing

configure: document that -Wwrite-strings is missing

Quote from GCC manual:

   "-Wwrite-strings

    When compiling C, give string constants the type const
    char[length] so that copying the address of one into a non-const
    char * pointer produces a warning. These warnings help you find at
    compile time code that can try to write into a string constant, but
    only if you have been very careful about using const in declarations
    and prototypes. Otherwise, it is just a nuisance.

    This is why we did not make -Wall request these warnings."

While this statement may be true for old code, new code/APIs should
compile with the flag enabled.

At least now this ommission is documented and it is easy for developers
to activate it to check their code.