use absolute path for header/footer in Doxyfile

- makes it easier to copy Doxyfile for local Doxygen generation
  ie, for debugging and writing without processing everything

doc/Doxygen/Doxyfile

@@ -689,13 +689,13 @@ HTML_FILE_EXTENSION    = .html
 # each generated HTML page. If it is left blank doxygen will generate a
 # standard header.
 
-HTML_HEADER            = FoamHeader.html
+HTML_HEADER            = $(WM_PROJECT_DIR)/doc/Doxygen/FoamHeader.html
 
 # The HTML_FOOTER tag can be used to specify a personal HTML footer for
 # each generated HTML page. If it is left blank doxygen will generate a
 # standard footer.
 
-HTML_FOOTER            = FoamFooter.html
+HTML_FOOTER            = $(WM_PROJECT_DIR)/doc/Doxygen/FoamFooter.html
 
 # The HTML_STYLESHEET tag can be used to specify a user-defined cascading
 # style sheet that is used by each HTML page. It can be used to