patch-doc_CMakeLists.txt

--- doc/CMakeLists.txt.orig	2017-10-05 10:11:23 UTC
+++ doc/CMakeLists.txt
@@ -9,9 +9,9 @@ foreach (TOOL ${TOOLS})
 endforeach ()
 
 if (COMMON_INSTALL_PATH)
-  set (INSTALL_DOC_DIR "share/doc/GeographicLib")
+  set (INSTALL_DOC_DIR "share/doc/GeographicLib" CACHE STRING "")
 else ()
-  set (INSTALL_DOC_DIR "doc")
+  set (INSTALL_DOC_DIR "doc" CACHE STRING "")
 endif ()
 
 # Run doxygen, if available