Having CmakeListsCustom.txt in git defeats the purpose of its existence:

If it is used, there is a modified file in git.
Fix this by only including the file if it exists.
Other changes:
- Rename the existing Custom files to CMakeListsCustom.txt.example.
- Move the plugins custom file to the top level (same level as its
  including parent).
- Optionally allow a list of custom includes instead of the default one.

Change-Id: I8960eac6222f741c045055d43d1d5a2d4979caf6
Reviewed-on: https://code.wireshark.org/review/4163
Reviewed-by: Jörg Mayer <jmayer@loplof.de>

CMakeLists.txt

@@ -35,6 +35,9 @@ cmake_policy(SET CMP0015 NEW)
 #Where to find local cmake scripts
 set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/modules)
 
+include(UseCustomIncludes)
+ADD_CUSTOM_CMAKE_INCLUDE()
+
 # This cannot be implemented via option(...)
 if( NOT CMAKE_BUILD_TYPE )
 	set( CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING
@@ -772,7 +775,6 @@ ADD_CUSTOM_COMMAND(
 )
 
 if(ENABLE_PLUGINS)
-	include(plugins/CMakeListsCustom.txt)
 	set(HAVE_PLUGINS 1)
 	set(PLUGIN_DIR "${DATAFILE_DIR}/plugins/${CPACK_PACKAGE_VERSION}")
 	set(PLUGIN_INSTALL_DIR "${CMAKE_INSTALL_LIBDIR}/@CPACK_PACKAGE_NAME@/plugins/${CPACK_PACKAGE_VERSION}")

CMakeListsCustom.txt.example

@@ -25,4 +25,4 @@ set(CUSTOM_PLUGIN_SRC_DIR
 #	private_plugins/foo
 # or
 #   plugins/foo
-)
+)

cmake/modules/UseCustomIncludes.cmake

@@ -0,0 +1,20 @@
+#
+# - Include a custom CMake file relative to the current source directory.
+# - If no filename is provided, CMakeListsCustom.txt is used.
+#
+macro( ADD_CUSTOM_CMAKE_INCLUDE  )
+    if( ${ARGN} )
+        set( _file_list ${ARGN} )
+    else()
+        set( _file_list CMakeListsCustom.txt )
+    endif()
+    foreach (_include ${_file_list})
+        set( _include_file ${CMAKE_CURRENT_SOURCE_DIR}/${_include} )
+        if( EXISTS ${_include_file} )
+            message ( STATUS "Including ${_include_file}" )
+            include( ${_include_file} )
+        else()
+            message ( STATUS "No custom file found in ${CMAKE_CURRENT_SOURCE_DIR}" )
+        endif()
+    endforeach()
+endmacro()

epan/CMakeLists.txt

@@ -21,7 +21,7 @@
 
 include(UseABICheck)
 
-include(CMakeListsCustom.txt)
+ADD_CUSTOM_CMAKE_INCLUDE()
 
 if (HAVE_HFI_SECTION_INIT)
 	add_definitions(

ui/gtk/CMakeLists.txt

@@ -19,7 +19,7 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 #
 
-include(CMakeListsCustom.txt)
+ADD_CUSTOM_CMAKE_INCLUDE()
 
 set(WIRESHARK_GTK_SRC
 	about_dlg.c