The function `unsigned char *MD5(const unsigned char *d, size_t n,
unsigned char *md)` from OpenSSL is deprecated and therefore generates
lots of warnings in the build when compiling against OpenSSL 3.0.
Do not treat these warnings as errors, but keep the warning active. This
should be reverted once OpenSSL-3.0 support arrives.
Let's at least ensure that the source builds without warnings.
The build image used, Ubuntu, spits out quite some warnings about
ignored return values. Use '-Wno-unused-result' for now.
The configure options are chosen to resemble the default Debian version.
