fix doxygen's treatment of source paths (#403)

doxygen.headers-to-doxyfile creates a Doxygen configuration file and
populates its INPUT value with the paths to its sources. But when doing
so it does not take into account the full path to those sources, only
the part that is used in the source name without the grist. The
resulting file is only correct when either the provided path to a source is
absolute, or when the path is relative to CWD. Some globbing functions return
relative paths. And the sources for docs usually searched from the doc
subdir, not from CWD.

This change fixes that issue by having headers-to-doxyfile to prefix paths to
sources with their SEARCH variables.

src/tools/doxygen.jam

@@ -309,8 +309,14 @@ rule headers-to-doxyfile ( target : sources * : properties * )
     }
 
     local headers ;
-    for local header in $(sources:G=)
+    for local source in $(sources)
     {
+        local header = $(source:G=) ;
+        local root = [ on $(source) return $(SEARCH) ] ;
+        if $(root)
+        {
+            header = [ path.root $(header) $(root) ] ;
+        }
         header = [ translate-path $(header) ] ;
         headers += \"$(header)\" ;
     }