Add ExprTk_BUILD_EXAMPLES to build examples

Also move examples into an examples folder

examples/exprtk_simple_example_08.cpp

@@ -0,0 +1,89 @@
+/*
+ **************************************************************
+ *         C++ Mathematical Expression Toolkit Library        *
+ *                                                            *
+ * Simple Example 8                                           *
+ * 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>
+void composite()
+{
+   typedef exprtk::symbol_table<T>         symbol_table_t;
+   typedef exprtk::expression<T>           expression_t;
+   typedef exprtk::parser<T>               parser_t;
+   typedef exprtk::parser_error::type      error_t;
+   typedef exprtk::function_compositor<T>  compositor_t;
+   typedef typename compositor_t::function function_t;
+
+   compositor_t compositor;
+
+   T x = T(1);
+   T y = T(2);
+
+   symbol_table_t& symbol_table = compositor.symbol_table();
+   symbol_table.add_constants();
+   symbol_table.add_variable("x",x);
+   symbol_table.add_variable("y",y);
+
+   compositor
+      .add(
+      function_t("f","sin(x / pi)","x"));          // f(x) = sin(x / pi)
+
+   compositor
+      .add(
+      function_t("g","3*[f(x) + f(y)]","x","y"));  // g(x,y) = 3[f(x) + f(y)]
+
+   std::string expression_string = "g(1 + f(x), f(y) / 2)";
+
+   expression_t expression;
+   expression.register_symbol_table(symbol_table);
+
+   parser_t parser;
+
+   if (!parser.compile(expression_string,expression))
+   {
+      printf("Error: %s\tExpression: %s\n",
+             parser.error().c_str(),
+             expression_string.c_str());
+
+      for (std::size_t i = 0; i < parser.error_count(); ++i)
+      {
+         const error_t error = parser.get_error(i);
+
+         printf("Error: %02d  Position: %02d Type: [%14s] Msg: %s\tExpression: %s\n",
+                static_cast<unsigned int>(i),
+                static_cast<unsigned int>(error.token.position),
+                exprtk::parser_error::to_str(error.mode).c_str(),
+                error.diagnostic.c_str(),
+                expression_string.c_str());
+      }
+
+      return;
+   }
+
+   const T result = expression.value();
+
+   printf("%s = %e\n", expression_string.c_str(), result);
+}
+
+int main()
+{
+   composite<double>();
+   return 0;
+}