Simplify include path handling and support cross-platform path delimiters

- Simplified the include path processing logic by reducing unnecessary braces.
- Improved cross-platform compatibility by correctly handling non-standard path
  separators on Windows and Unix systems.
- Combined the default include path addition and environment variable parsing
  into a more concise and efficient implementation.

Author: AhmedFatthy1040

src/xinterpreter.cpp

@@ -15,6 +15,7 @@
 #include "xeus-cpp/xinterpreter.hpp"
 #include "xeus-cpp/xmagics.hpp"
 
+#include <cstdlib>    // for std::getenv
 #include <cstring>    // for std::strlen
 #include <sstream>    // for std::istringstream
 #include <string>     // for std::getline
@@ -364,25 +365,23 @@ __get_cxx_version ()
         // Add the standard include path
         Cpp::AddIncludePath((xeus::prefix_path() + "/include/").c_str());
 
-        // Get include paths from environment variable
+        // Get include paths from environment variable and use empty string if not set
         const char* non_standard_paths = std::getenv("XEUS_SEARCH_PATH");
-        if (!non_standard_paths) {
+        if (!non_standard_paths)
             non_standard_paths = "";
-        }
 
-        if (std::strlen(non_standard_paths) > 0)
-        {
-            // Split the paths by colon ':' and add each one
-            std::istringstream stream(non_standard_paths);
-            std::string path;
-            while (std::getline(stream, path, ':'))
-            {
-                if (!path.empty())
-                {
-                    Cpp::AddIncludePath(path.c_str());
-                }
-            }
-        }
+#ifdef _WIN32
+        const char path_separator = ';';
+#else
+        const char path_separator = ':';
+#endif
+
+        // Split and add each non-empty path
+        std::istringstream stream(non_standard_paths);
+        std::string path;
+        while (std::getline(stream, path, path_separator))
+            if (!path.empty())
+                Cpp::AddIncludePath(path.c_str());
     }
 
     void interpreter::init_preamble()