Merge #13899: build: Enable -Wredundant-decls where available. Remove redundant redeclarations.

d56b73f217 Remove redundant extern (practicalswift)
f04bb1361c Enable -Wredundant-decls (gcc) if available (practicalswift)
a9e90e5002 Remove redundant redeclaration of rescanblockchain(...) in same scope (practicalswift)

Pull request description:

  Remove redundant redeclaration of `rescanblockchain` and enable `-Wredundant-decls` (gcc) where available to avoid accidental redundant redeclarations.

  ```
   CXX      wallet/libbitcoin_wallet_a-rpcwallet.o
  wallet/rpcwallet.cpp:4764:17: warning: redundant redeclaration of ‘UniValue rescanblockchain(const JSONRPCRequest&)’ in same scope [-Wredundant-decls]
   extern UniValue rescanblockchain(const JSONRPCRequest& request);
                   ^~~~~~~~~~~~~~~~
  wallet/rpcwallet.cpp:3929:10: note: previous declaration of ‘UniValue rescanblockchain(const JSONRPCRequest&)’
   UniValue rescanblockchain(const JSONRPCRequest& request)
            ^~~~~~~~~~~~~~~~
  ```

Tree-SHA512: b9af95fa53f494c3f6702e485956b66b042d2ff7578b4a53bf28e91aa844cdcf5d7ac3e2e710948eed566007324e81317304b8eabf2d4ea284cd6acd77f8ffcd

configure.ac

@@ -303,6 +303,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wformat-security],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wformat-security"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wthread-safety-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wthread-safety-analysis"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wrange-loop-analysis],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wrange-loop-analysis"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])
 
   ## Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   ## unknown options if any other warning is produced. Test the -Wfoo case, and

src/wallet/rpcwallet.cpp

@@ -4751,17 +4751,16 @@ UniValue walletcreatefundedpsbt(const JSONRPCRequest& request)
     return result;
 }
 
-extern UniValue abortrescan(const JSONRPCRequest& request); // in rpcdump.cpp
-extern UniValue dumpprivkey(const JSONRPCRequest& request); // in rpcdump.cpp
-extern UniValue importprivkey(const JSONRPCRequest& request);
-extern UniValue importaddress(const JSONRPCRequest& request);
-extern UniValue importpubkey(const JSONRPCRequest& request);
-extern UniValue dumpwallet(const JSONRPCRequest& request);
-extern UniValue importwallet(const JSONRPCRequest& request);
-extern UniValue importprunedfunds(const JSONRPCRequest& request);
-extern UniValue removeprunedfunds(const JSONRPCRequest& request);
-extern UniValue importmulti(const JSONRPCRequest& request);
-extern UniValue rescanblockchain(const JSONRPCRequest& request);
+UniValue abortrescan(const JSONRPCRequest& request); // in rpcdump.cpp
+UniValue dumpprivkey(const JSONRPCRequest& request); // in rpcdump.cpp
+UniValue importprivkey(const JSONRPCRequest& request);
+UniValue importaddress(const JSONRPCRequest& request);
+UniValue importpubkey(const JSONRPCRequest& request);
+UniValue dumpwallet(const JSONRPCRequest& request);
+UniValue importwallet(const JSONRPCRequest& request);
+UniValue importprunedfunds(const JSONRPCRequest& request);
+UniValue removeprunedfunds(const JSONRPCRequest& request);
+UniValue importmulti(const JSONRPCRequest& request);
 
 static const CRPCCommand commands[] =
 { //  category              name                                actor (function)                argNames