FindPkgConfig: Tolerate PKG_CONFIG_SYSTEM_LIBRARY_PATH in environment

Tell `pkg-config --libs` not to filter out `-L` flags for entries of
`PKG_CONFIG_SYSTEM_LIBRARY_PATH` (and `LIBRARY_PATH` for `pkgconf`).
We should always search everywhere the `.pc` file expects.

Fixes: #22148

Modules/FindPkgConfig.cmake

@@ -403,6 +403,14 @@ macro(_pkg_set_path_internal)
     unset(_lib_dirs)
     unset(_pkgconfig_path)
   endif()
+
+  # Tell pkg-config not to strip any -L paths so we can search them all.
+  if(DEFINED ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS})
+    set(_pkgconfig_allow_system_libs_old "$ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS}")
+  else()
+    unset(_pkgconfig_allow_system_libs_old)
+  endif()
+  set(ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS} 0)
 endmacro()
 
 macro(_pkg_restore_path_internal)
@@ -410,6 +418,10 @@ macro(_pkg_restore_path_internal)
     # Restore the environment variable
     set(ENV{PKG_CONFIG_PATH} "${_pkgconfig_path_old}")
   endif()
+  if(DEFINED _pkgconfig_allow_system_libs_old)
+    set(ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS} "${_pkgconfig_allow_system_libs_old}")
+    unset(_pkgconfig_allow_system_libs_old)
+  endif()
 
   unset(_extra_paths)
   unset(_pkgconfig_path_old)

Tests/RunCMake/FindPkgConfig/FindPkgConfig_LIBRARY_PATH-stdout.txt

@@ -0,0 +1,3 @@
+-- ZOT_LIBRARIES='zot'
+-- ZOT_LINK_LIBRARIES='[^']*/Tests/RunCMake/FindPkgConfig/FindPkgConfig_LIBRARY_PATH-build/zot/lib/prefix-zot-suffix'
+-- ZOT_LDFLAGS='-L[^']*/Tests/RunCMake/FindPkgConfig/FindPkgConfig_LIBRARY_PATH-build/zot/lib;-lzot'

Tests/RunCMake/FindPkgConfig/FindPkgConfig_LIBRARY_PATH.cmake

@@ -0,0 +1,29 @@
+find_package(PkgConfig REQUIRED)
+
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/zot/lib/pkgconfig/zot.pc" "
+prefix=${CMAKE_CURRENT_BINARY_DIR}/zot
+libdir=\${prefix}/lib
+
+Name: Zot
+Description: Dummy packaget to test LIBRARY_DIR support
+Version: 1.0
+Libs: -L\${libdir} -lzot
+")
+
+# Create a "library" file to find in libdir.
+set(CMAKE_FIND_LIBRARY_PREFIXES "prefix-")
+set(CMAKE_FIND_LIBRARY_SUFFIXES "-suffix")
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/zot/lib/prefix-zot-suffix")
+
+# 'pkg-config --libs' drops -L flags in PKG_CONFIG_SYSTEM_LIBRARY_PATH by default.
+set(ENV{PKG_CONFIG_SYSTEM_LIBRARY_PATH} "${CMAKE_CURRENT_BINARY_DIR}/zot/lib")
+
+# 'pkgconf --libs' also drops -L flags in LIBRARY_PATH by default.
+set(ENV{LIBRARY_PATH}                   "${CMAKE_CURRENT_BINARY_DIR}/zot/lib")
+
+set(ENV{PKG_CONFIG_PATH}                "${CMAKE_CURRENT_BINARY_DIR}/zot/lib/pkgconfig")
+pkg_check_modules(ZOT REQUIRED zot)
+
+message(STATUS "ZOT_LIBRARIES='${ZOT_LIBRARIES}'")
+message(STATUS "ZOT_LINK_LIBRARIES='${ZOT_LINK_LIBRARIES}'")
+message(STATUS "ZOT_LDFLAGS='${ZOT_LDFLAGS}'")

Tests/RunCMake/FindPkgConfig/RunCMakeTest.cmake

@@ -31,4 +31,8 @@ if (PKG_CONFIG_FOUND)
   run_cmake(FindPkgConfig_VERSION_OPERATORS)
   run_cmake(FindPkgConfig_GET_MATCHING_MODULE_NAME)
   run_cmake(FindPkgConfig_empty_target)
+
+  if(NOT RunCMake_BINARY_DIR MATCHES " ")
+    run_cmake(FindPkgConfig_LIBRARY_PATH)
+  endif()
 endif ()