This flag was originally not added because of crypto functions,
but together with -lssp in 460879.

Advapi32 is one of the libs that are automatically linked in by
the mingw compiler/linker driver.

Adding it manually here causes issues with linking libupnp_plugin.la
with lld. Libupnp contains md5 functions, and advapi32 also contains
functions with the same names. If -ladvapi32 is specified early in the
link command, before -lupnp, the conflicts arise. (The same conflicts
don't occur with binutils ld though, since it searches libraries
in a slightly different order than lld.)

The conflict can be avoided by fixing one inconsistency in mingw-w64
though, but then libupnp will use MD5 functions from advapi32.dll
(with an ABI not guaranteed to match the ones it expected) instead
of the ones bundled in libupnp.