FindPkgConfig: Recheck pkg-config on parameter change.

Currently, once pkg_check_modules succeeds, it will never call
_pkg_check_modules_internal again.  That means that if the parameters
to pkg_check_modules are changed, cmake will be called to reconfigure,
but nothing will change.  This change is to store the full string of
arguments to pkg_check_modules and override the FOUND optimization so
that the arguments are reevaluated when modified.

Help/release/dev/pkg-config-recheck.rst

@@ -0,0 +1,7 @@
+pkg-config-recheck
+------------------
+
+* Calls to the :module:`FindPkgConfig` module :command:`pkg_check_modules`
+  command following a successful call learned to re-evaluate the cached values
+  for a given prefix after changes to the parameters to the command for that
+  prefix.

Modules/FindPkgConfig.cmake

@@ -585,10 +585,13 @@ endmacro()
 macro(pkg_check_modules _prefix _module0)
   _pkgconfig_parse_options(_pkg_modules _pkg_is_required _pkg_is_silent _no_cmake_path _no_cmake_environment_path _imp_target "${_module0}" ${ARGN})
   # check cached value
-  if (NOT DEFINED __pkg_config_checked_${_prefix} OR __pkg_config_checked_${_prefix} LESS ${PKG_CONFIG_VERSION} OR NOT ${_prefix}_FOUND)
+  if (NOT DEFINED __pkg_config_checked_${_prefix} OR __pkg_config_checked_${_prefix} LESS ${PKG_CONFIG_VERSION} OR NOT ${_prefix}_FOUND OR NOT "${__pkg_config_arguments_${_prefix}}" STREQUAL "${_module0};${ARGN}")
     _pkg_check_modules_internal("${_pkg_is_required}" "${_pkg_is_silent}" ${_no_cmake_path} ${_no_cmake_environment_path} ${_imp_target} "${_prefix}" ${_pkg_modules})
 
     _pkgconfig_set(__pkg_config_checked_${_prefix} ${PKG_CONFIG_VERSION})
+    if (${_prefix}_FOUND)
+      _pkgconfig_set(__pkg_config_arguments_${_prefix} "${_module0};${ARGN}")
+    endif()
   elseif (${_prefix}_FOUND AND ${_imp_target})
     _pkg_create_imp_target("${_prefix}" _no_cmake_path _no_cmake_environment_path)
   endif()