chore: Fix compilation error with clang-20 and cleanup (#5543)

Removes clutter for old compilers, defaults to non-unity builds in cmake to match conanfile.py, and workaround for clang-20 compilation errors.

include/xrpl/basics/Expected.h

@@ -22,8 +22,18 @@
 
 #include <xrpl/basics/contract.h>
 
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated"
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
 #include <boost/outcome.hpp>
 
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 #include <stdexcept>
 
 namespace ripple {

include/xrpl/beast/hash/hash_append.h

@@ -24,13 +24,38 @@
 #include <boost/container/flat_set.hpp>
 #include <boost/endian/conversion.hpp>
 
+/*
+
+Workaround for overzealous clang warning, which trips on libstdc++ headers
+
+  In file included from
+  /usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/stl_algo.h:61:
+  /usr/lib/gcc/x86_64-linux-gnu/12/../../../../include/c++/12/bits/stl_tempbuf.h:263:8:
+  error: 'get_temporary_buffer<std::pair<ripple::Quality, const
+  std::vector<std::unique_ptr<ripple::Step>> *>>' is deprecated
+  [-Werror,-Wdeprecated-declarations] 263 |
+  std::get_temporary_buffer<value_type>(_M_original_len));
+       ^
+*/
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated"
+#pragma clang diagnostic ignored "-Wdeprecated-declarations"
+#endif
+
+#include <functional>
+#include <memory>
+
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
 #include <array>
 #include <chrono>
 #include <cstdint>
 #include <cstring>
-#include <functional>
 #include <map>
-#include <memory>
 #include <set>
 #include <string>
 #include <system_error>