String literals have the type 'char[N]' by default. Allthough they are
not really 'const', modifying a string literal is undefined behaviour.
Therefore, to avoid mistakes, libdivecomputer uses the -Wwrite-strings
compiler option to change the default type to 'const char[N]'.
The cast that triggers the -Wcast-qual warning can be avoided by using a
character array instead of string literal.
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