Revert "Delete and re-add libpqxx-7.7.3 due to weird corruption."

This reverts commit e96515433d.

ext/libpqxx-7.7.3/config-tests/concepts.cxx

@@ -0,0 +1,21 @@
+// Test for concepts support.  Not just the language feature; also headers.
+#include <iostream>
+#include <ranges>
+#include <vector>
+
+
+template<typename T>
+concept Foo = std::ranges::input_range<T>;
+
+
+template<Foo F> auto foo(F const &r)
+{
+  return std::distance(std::begin(r), std::end(r));
+}
+
+
+int main()
+{
+  std::vector<int> const v{1, 2, 3};
+  std::cout << foo(v) << '\n';
+}