src: CppAD 2024 wants ordered complex numbers

Fix compilation error.

Define lexical order for complex numbers.

src/common.hpp

@@ -63,6 +63,10 @@ namespace CppAD {
     COMMON_HPP_DECLARE_BINOP(*);
     COMMON_HPP_DECLARE_BINOP(/);
 #undef COMMON_HPP_DECLARE_BINOP
+
+    /* CppAD needs comparisons; use lexical order for complex */
+    bool operator <=(const Complex& a, const Complex& b) { return a.real() == b.real() ? (a.imag() <= b.imag()) : (a.real() <= b.real()); }
+    bool operator <(const Complex& a, const Complex& b) { return a.real() == b.real() ? (a.imag() < b.imag()) : (a.real() < b.real()); }
 }
 
 #endif