Switch XEUS_SEARCH_PATH from compile-time to runtime

This change:
1. Replaces compile-time XEUS_SEARCH_PATH define with runtime environment variable
2. Removes CMake definition to eliminate redefinition warnings
3. Adds test to verify non-standard include path functionality
4. Makes include path configuration more flexible (can be changed without recompiling)

Resolves compiler-research#175

CMakeLists.txt

@@ -50,6 +50,8 @@ message(STATUS "Building xeus-cpp v${${PROJECT_NAME}_VERSION}")
 # Build options
 # =============
 
+# Note: XEUS_SEARCH_PATH is now handled at runtime through environment variables
+
 option(XEUS_CPP_BUILD_STATIC "Build xeus-cpp static library" ON)
 option(XEUS_CPP_BUILD_SHARED "Split xcpp build into executable and library" ON)
 option(XEUS_CPP_BUILD_EXECUTABLE "Build the xcpp executable" ON)
@@ -404,7 +406,21 @@ endif()
 # =====
 
 if(XEUS_CPP_BUILD_TESTS)
+    enable_testing()
     add_subdirectory(test)
+
+    # Find doctest package
+    find_package(doctest REQUIRED)
+
+    # Test for non-standard include paths
+    add_executable(test_include_paths test/test_include_paths.cpp)
+    target_link_libraries(test_include_paths PRIVATE doctest::doctest)
+    target_include_directories(test_include_paths PRIVATE 
+        ${CMAKE_SOURCE_DIR}/test/custom_includes
+        ${DOCTEST_INCLUDE_DIR}
+    )
+    target_compile_definitions(test_include_paths PRIVATE DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN)
+    add_test(NAME test_include_paths COMMAND test_include_paths)
 endif()
 
 # Installation

src/xinterpreter.cpp

@@ -360,10 +360,13 @@ __get_cxx_version ()
         // Add the standard include path
         Cpp::AddIncludePath((xeus::prefix_path() + "/include/").c_str());
 
-        // Add non-standard include paths from XEUS_SEARCH_PATH
-        const char* non_standard_paths = XEUS_SEARCH_PATH;
+        // Get include paths from environment variable
+        const char* non_standard_paths = std::getenv("XEUS_SEARCH_PATH");
+        if (!non_standard_paths) {
+            non_standard_paths = "";
+        }
 
-        if (non_standard_paths && std::strlen(non_standard_paths) > 0)
+        if (std::strlen(non_standard_paths) > 0)
         {
             // Split the paths by colon ':' and add each one
             std::istringstream stream(non_standard_paths);

test/custom_includes/test_header.hpp

@@ -0,0 +1,8 @@
+#ifndef TEST_HEADER_HPP
+#define TEST_HEADER_HPP
+
+namespace test_ns {
+    constexpr int test_value = 42;
+}
+
+#endif

test/test_include_paths.cpp

@@ -0,0 +1,8 @@
+#include <doctest/doctest.h>
+#include <string>
+#include "test_header.hpp"
+
+TEST_CASE("Test non-standard include paths")
+{
+    CHECK(test_ns::test_value == 42);
+}