PkgC: Allow system paths during IMPORT, POPULATE

Fixes: #27138

Source/cmCMakePkgConfigCommand.cxx

@@ -911,6 +911,9 @@ std::pair<bool, bool> PopulatePCTarget(PopulateArguments& args,
   auto& pcEnv = maybeEnv->first;
   auto& imEnv = maybeEnv->second;
 
+  pcEnv.AllowSysCflags = true;
+  pcEnv.AllowSysLibs = true;
+
   pkgProviders providers;
   if (args.Providers) {
     for (auto const& provider_str : *args.Providers) {

Tests/RunCMake/cmake_pkg_config/ImportSystem-check.cmake

@@ -0,0 +1,11 @@
+set(expected
+"Include Directories: /TestDirectories/Include
+Link Directories: /TestDirectories/Library
+"
+)
+
+file(READ "${RunCMake_TEST_BINARY_DIR}/import-system.txt" actual)
+
+if(NOT(expected STREQUAL actual))
+  set(RunCMake_TEST_FAILED "cmake_pkg_config import-system.txt does not match expected:\n${actual}")
+endif()

Tests/RunCMake/cmake_pkg_config/ImportSystem.cmake

@@ -0,0 +1,12 @@
+set(CMAKE_PKG_CONFIG_SYS_INCLUDE_DIRS /TestDirectories/Include)
+set(CMAKE_PKG_CONFIG_SYS_LIB_DIRS /TestDirectories/Library)
+
+cmake_pkg_config(IMPORT import-simple REQUIRED)
+
+file(GENERATE
+  OUTPUT import-system.txt
+  CONTENT
+"Include Directories: $<TARGET_PROPERTY:PkgConfig::import-simple,INTERFACE_INCLUDE_DIRECTORIES>
+Link Directories: $<TARGET_PROPERTY:PkgConfig::import-simple,INTERFACE_LINK_DIRECTORIES>
+"
+)

Tests/RunCMake/cmake_pkg_config/RunCMakeTest.cmake

@@ -20,6 +20,7 @@ run_cmake(ImportName)
 run_cmake(ImportPrefix)
 run_cmake(ImportRequires)
 run_cmake(ImportSimple)
+run_cmake(ImportSystem)
 run_cmake(ImportTransitiveFail)
 run_cmake(ImportTransitiveVersion)
 run_cmake(ImportTransitiveVersionFail)