cmake: fix regression in finding wx from 2efcb62

Clear FindwxWidgets module variables from the cache after each
unsuccessful try with WX_CONFIG_EXECUTABLE set, otherwise the succeeding
find_package(wxWidgets) call with WX_CONFIG_EXECUTABLE unset will fail,
as it does with cmake on gentoo.

src/wx/CMakeLists.txt

@@ -65,6 +65,14 @@ function(normalize_wx_paths)
     endif()
 endfunction()
 
+function(clear_wx_vars)
+    unset(wxWidgets_FOUND        PARENT_SCOPE CACHE FORCE)
+    unset(wxWidgets_INCLUDE_DIRS PARENT_SCOPE CACHE FORCE)
+    unset(wxWidgets_LIBRARIES    PARENT_SCOPE CACHE FORCE)
+    unset(wxWidgets_LIBRARY_DIRS PARENT_SCOPE CACHE FORCE)
+    unset(wxWidgets_CXX_FLAGS    PARENT_SCOPE CACHE FORCE)
+endfunction()
+
 if(CMAKE_PREFIX_PATH)
     set(wxWidgets_CONFIG_OPTIONS "--prefix=${CMAKE_PREFIX_PATH}")
 endif()
@@ -87,6 +95,7 @@ if("$ENV{WX_CONFIG}" STREQUAL "")
         if(wxWidgets_FOUND)
             break()
         endif()
+        clear_wx_vars()
     endforeach()
 
     if(NOT wxWidgets_FOUND)