[gtk-doc/tintou/cmake-no-dup-flags] cmake: Avoid duplicate flags

[Corentin Noël <corentinnoel src gnome org>]

commit 0a7a1a8dbda64543832209b8c229d98aff75a847
Author: Corentin Noël <corentin elementary io>
Date:   Thu Mar 14 07:34:33 2019 +0100

    cmake: Avoid duplicate flags

 buildsystems/cmake/GtkDocConfig.cmake | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/buildsystems/cmake/GtkDocConfig.cmake b/buildsystems/cmake/GtkDocConfig.cmake
index 57b2aac..c26e679 100644
--- a/buildsystems/cmake/GtkDocConfig.cmake
+++ b/buildsystems/cmake/GtkDocConfig.cmake
@@ -158,6 +158,15 @@ function(gtk_doc_add_module _doc_prefix)
         list(APPEND _extra_ldpath ${library_dir})
     endforeach()
 
+    list(REMOVE_DUPLICATES _extra_cflags)
+    list(SORT _extra_cflags)
+
+    list(REMOVE_DUPLICATES _extra_ldflags)
+    list(SORT _extra_ldflags)
+
+    list(REMOVE_DUPLICATES _extra_ldpath)
+    list(SORT _extra_ldpath)
+
     # a directory to store output.
     set(_output_dir "${CMAKE_CURRENT_BINARY_DIR}/${_doc_prefix}")
     set(_output_dir_stamp "${_output_dir}/dir.stamp")