Updates to build system to better support automake-1.5

- automake-1.4 sets default values for INCLUDES which we can't
  overwrite.
- automake-1.5 renames this to DEFAULT_INCLUDES and leaves INCLUDES
  open for the developer to use.

Thus for automake-1.4 we are forced to 'append' to INCLUDES and in
automake-1.5 we can just set the value to whatever we like.
Unfortunately, the behaviors of the two versions are mutually
incompatible.

The solution I am committing now works for both versions but
automake-1.5 generates a lot of spurious warning messages that are
annoying, but not fatal.

simgear/xml/Makefile.am

@@ -3,7 +3,7 @@ includedir = @includedir@/xml
 if HAVE_ZLIB
 ZLIB_INCL =
 else
-XLIB_INCL = -I$(top_srcdir)/src/zlib
+ZLIB_INCL = -I$(top_srcdir)/src/zlib
 endif
 
 lib_LIBRARIES = libsgxml.a
@@ -24,4 +24,8 @@ libsgxml_a_SOURCES = \
 	xmltok.h xmltok.c \
 	xmltok_impl.h
 
+if OLD_AUTOMAKE
 INCLUDES += -I$(top_srcdir) $(ZLIB_INCL)
+else
+INCLUDES = -I$(top_srcdir) $(ZLIB_INCL)
+endif