Add ExprTk_BUILD_EXAMPLES to build examples

Also move examples into an examples folder

examples/exprtk_simple_example_05.cpp

@@ -0,0 +1,80 @@
+/*
+ **************************************************************
+ *         C++ Mathematical Expression Toolkit Library        *
+ *                                                            *
+ * Simple Example 5                                           *
+ * Author: Arash Partow (1999-2023)                           *
+ * URL: https://www.partow.net/programming/exprtk/index.html  *
+ *                                                            *
+ * Copyright notice:                                          *
+ * Free use of the Mathematical Expression Toolkit Library is *
+ * permitted under the guidelines and in accordance with the  *
+ * most current version of the MIT License.                   *
+ * https://www.opensource.org/licenses/MIT                    *
+ *                                                            *
+ **************************************************************
+*/
+
+
+#include <cstdio>
+#include <string>
+
+#include "exprtk.hpp"
+
+template <typename T>
+struct myfunc : public exprtk::ifunction<T>
+{
+   using exprtk::ifunction<T>::operator();
+
+   myfunc()
+   : exprtk::ifunction<T>(2)
+   { exprtk::disable_has_side_effects(*this); }
+
+   inline T operator()(const T& v1, const T& v2)
+   {
+      return T(1) + (v1 * v2) / T(3);
+   }
+};
+
+template <typename T>
+inline T myotherfunc(T v0, T v1, T v2)
+{
+   return std::abs(v0 - v1) * v2;
+}
+
+template <typename T>
+void custom_function()
+{
+   typedef exprtk::symbol_table<T> symbol_table_t;
+   typedef exprtk::expression<T>   expression_t;
+   typedef exprtk::parser<T>       parser_t;
+
+   const std::string expression_string =
+                  "myfunc(sin(x / pi), otherfunc(3 * y, x / 2, x * y))";
+
+   T x = T(1);
+   T y = T(2);
+   myfunc<T> mf;
+
+   symbol_table_t symbol_table;
+   symbol_table.add_variable("x",x);
+   symbol_table.add_variable("y",y);
+   symbol_table.add_function("myfunc",mf);
+   symbol_table.add_function("otherfunc",myotherfunc);
+   symbol_table.add_constants();
+
+   expression_t expression;
+   expression.register_symbol_table(symbol_table);
+
+   parser_t parser;
+   parser.compile(expression_string,expression);
+
+   const T result = expression.value();
+   printf("Result: %10.5f\n",result);
+}
+
+int main()
+{
+   custom_function<double>();
+   return 0;
+}