This is just not needed, the variable value is the same as its cached
value (if any) at this point anyhow.

This can be useful to check how the library was configured later.

Just as with the global summary, only show the message once or if it
changes later, otherwise it's displayed every time CMake is rerun, even
if there have been no changes.

Minor CMake build summary improvements.

See #1299.

It is convenient to allow specifying this option as part of the
connection string itself rather than requiring setting it using
set_option(), e.g. like this it can be used in the connection string
used by the CI service without requiring any changes in the code, so
support giving it in this way.

See #1300.

Use LoadLibraryEx(LOAD_WITH_ALTERED_SEARCH_PATH) to load backend
dependencies from the directory containing the backend DLL itself first,
while still looking in the standard search path if the dependency is not
found there.

This is important to prevent the DLLs found in the application
directory, which is searched before the directory containing the DLL in
the standard search path, from being loaded instead and breaking SOCI
backend.

The motivating example is using SOCI from Microsoft Excel add-in, which
would load OpenSSL libcrypto-3-x64.dll from EXCEL.EXE directory, which
contains an old version of this DLL incompatible with recent libpq.dll
versions and so preventing PostgreSQL DLL, and hence SOCI Postgres
plugin, from being loaded.

See #1301.