Allow using vendored tinyxml2 for XMLRPC

By default, builds will still not have XMLRPC enabled at all and the
configure flag `--with-xmlrpc-tinyxml2` must be specified. If both
xmlrpc-c and tinyxml2 are specified, xmlrpc-c takes precedence.

Basic benchmarks indicate tinyxml2 is 2x faster for small
requests/responses, and that only increases as response sizes get
larger.

src/utils/base64.h

@@ -0,0 +1,59 @@
+#include <string>
+
+#include <torrent/exceptions.h>
+
+namespace utils {
+constexpr static char padCharacter = '=';
+static std::string
+base64decode(const std::string& input) {
+  if (input.length() % 4) // Sanity check
+    throw torrent::input_error("Invalid base64.");
+  size_t padding = 0;
+  if (input.length()) {
+    if (input[input.length() - 1] == padCharacter)
+      padding++;
+    if (input[input.length() - 2] == padCharacter)
+      padding++;
+  }
+  // Setup a vector to hold the result
+  std::string decodedBytes;
+  decodedBytes.reserve(((input.length() / 4) * 3) - padding);
+  long                             temp   = 0; // Holds decoded quanta
+  std::string::const_iterator cursor = input.begin();
+  while (cursor < input.end()) {
+    for (size_t quantumPosition = 0; quantumPosition < 4; quantumPosition++) {
+      temp <<= 6;
+      if (*cursor >= 0x41 && *cursor <= 0x5A) // This area will need tweaking if
+        temp |= *cursor - 0x41; // you are using an alternate alphabet
+      else if (*cursor >= 0x61 && *cursor <= 0x7A)
+        temp |= *cursor - 0x47;
+      else if (*cursor >= 0x30 && *cursor <= 0x39)
+        temp |= *cursor + 0x04;
+      else if (*cursor == 0x2B)
+        temp |= 0x3E; // change to 0x2D for URL alphabet
+      else if (*cursor == 0x2F)
+        temp |= 0x3F;                   // change to 0x5F for URL alphabet
+      else if (*cursor == padCharacter) // pad
+      {
+        switch (input.end() - cursor) {
+          case 1: // One pad character
+            decodedBytes.push_back((temp >> 16) & 0x000000FF);
+            decodedBytes.push_back((temp >> 8) & 0x000000FF);
+            return decodedBytes;
+          case 2: // Two pad characters
+            decodedBytes.push_back((temp >> 10) & 0x000000FF);
+            return decodedBytes;
+          default:
+            throw torrent::input_error("Invalid padding in base64.");
+        }
+      } else
+        throw torrent::input_error("Invalid character in base64.");
+      cursor++;
+    }
+    decodedBytes.push_back((temp >> 16) & 0x000000FF);
+    decodedBytes.push_back((temp >> 8) & 0x000000FF);
+    decodedBytes.push_back((temp)&0x000000FF);
+  }
+  return decodedBytes;
+}
+}