Use relative path in Doxygen STRIP_FROM_PATH

This seems better than requiring yet another environment variable to be
defined, and should work reliably as Doxygen is always run from
docs/doxygen directory.

Leave WXWIDGETS in STRIP_FROM_INC_PATH which doesn't seem to be used
anywhere anyhow -- and so, perhaps, should be removed entirely?

docs/doxygen/Doxyfile

@@ -19,7 +19,7 @@ ABBREVIATE_BRIEF       =
 ALWAYS_DETAILED_SEC    = YES # Default: NO
 INLINE_INHERITED_MEMB  = NO
 FULL_PATH_NAMES        = YES
-STRIP_FROM_PATH        = "$(WXWIDGETS)/"
+STRIP_FROM_PATH        = ../..
 STRIP_FROM_INC_PATH    = "$(WXWIDGETS)/include/" \
                          "$(WXWIDGETS)/interface/"
 SHORT_NAMES            = NO