FindPkgConfig: add a command to query arbitrary variables

Help/release/dev/pkg-config-variable-function.rst

+pkg-config-variable-function
+----------------------------
+
+* The :module:`FindPkgConfig` learned a new :command:`pkg_get_variable`
+  command which may be used to query for arbitrary variables from a package
+  (such as for related tools or data and plugin install paths).

Modules/FindPkgConfig.cmake

@@ -91,6 +91,26 @@ macro(_pkgconfig_invoke _pkglist _prefix _varname _regexp)
   endif()
 endmacro()
 
+#[========================================[.rst:
+.. command:: pkg_get_variable
+
+  Retrieves the value of a variable from a package::
+
+    pkg_get_variable(<RESULT> <MODULE> <VARIABLE>)
+
+  For example:
+
+  .. code-block:: cmake
+
+    pkg_get_variable(GI_GIRDIR gobject-introspection-1.0 girdir)
+#]========================================]
+function (pkg_get_variable result pkg variable)
+  _pkgconfig_invoke("${pkg}" "prefix" "result" "" "--variable=${variable}")
+  set("${result}"
+    "${prefix_result}"
+    PARENT_SCOPE)
+endfunction ()
+
 # Invokes pkgconfig two times; once without '--static' and once with
 # '--static'
 macro(_pkgconfig_invoke_dyn _pkglist _prefix _varname cleanup_regexp)
@@ -356,9 +376,9 @@ macro(_pkg_check_modules_internal _is_required _is_silent _no_cmake_path _no_cma
         endif()
 
         _pkgconfig_invoke(${_pkg_check_modules_pkg} "${_pkg_check_prefix}" VERSION    ""   --modversion )
-        _pkgconfig_invoke(${_pkg_check_modules_pkg} "${_pkg_check_prefix}" PREFIX     ""   --variable=prefix )
-        _pkgconfig_invoke(${_pkg_check_modules_pkg} "${_pkg_check_prefix}" INCLUDEDIR ""   --variable=includedir )
-        _pkgconfig_invoke(${_pkg_check_modules_pkg} "${_pkg_check_prefix}" LIBDIR     ""   --variable=libdir )
+        pkg_get_variable("${_pkg_check_prefix}_PREFIX" ${_pkg_check_modules_pkg} "prefix")
+        pkg_get_variable("${_pkg_check_prefix}_INCLUDEDIR" ${_pkg_check_modules_pkg} "includedir")
+        pkg_get_variable("${_pkg_check_prefix}_LIBDIR" ${_pkg_check_modules_pkg} "libdir")
 
         if (NOT ${_is_silent})
           message(STATUS "  Found ${_pkg_check_modules_pkg}, version ${_pkgconfig_VERSION}")

Tests/RunCMake/FindPkgConfig/FindPkgConfig_GET_VARIABLE-stdout.txt

+-- g_ir_scanner: .*/g-ir-scanner

Tests/RunCMake/FindPkgConfig/FindPkgConfig_GET_VARIABLE.cmake

+find_package(PkgConfig REQUIRED)
+pkg_check_modules(GOBJECT_INTROSPECTION QUIET gobject-introspection-1.0)
+
+if (GOBJECT_INTROSPECTION_FOUND)
+  pkg_get_variable(g_ir_scanner gobject-introspection-1.0 g_ir_scanner)
+  message(STATUS "g_ir_scanner: ${g_ir_scanner}")
+else ()
+  message(STATUS "g_ir_scanner: skipping test; gobject-introspection-1.0 not found /g-ir-scanner")
+endif ()

Tests/RunCMake/FindPkgConfig/RunCMakeTest.cmake

@@ -9,3 +9,9 @@ if(APPLE)
   run_cmake(FindPkgConfig_CMAKE_FRAMEWORK_PATH)
   run_cmake(FindPkgConfig_CMAKE_APPBUNDLE_PATH)
 endif()
+
+# We need a real pkg-config to run the test for get_variable.
+find_package(PkgConfig)
+if (PKG_CONFIG_FOUND)
+  run_cmake(FindPkgConfig_GET_VARIABLE)
+endif ()