gnulib added a new syntax-check test: use $(VAR), not @VAR@

The latter is not officially "wrong", but *is* terribly anachronistic.
I think automake documentation or comments call that syntax obsolescent.
* cfg.mk (_makefile_at_at_check_exceptions): Exempt @SCHEMADIR@
and @SYSCONFDIR@ uses -- there are no Makefile variables for those.
* docs/Makefile.am: Use $(INSTALL), not @INSTALL@.
* examples/dominfo/Makefile.am: Similar.
* examples/domsuspend/Makefile.am: Similar.
* proxy/Makefile.am: Similar.
* python/Makefile.am: Similar.
* python/tests/Makefile.am: Similar.
* src/Makefile.am: Similar.
* tests/Makefile.am: Similar.

proxy/Makefile.am

@@ -7,7 +7,7 @@ INCLUDES = -I$(top_srcdir)/gnulib/lib -I../gnulib/lib \
            -I@top_srcdir@/src/util \
            -I@top_srcdir@/src/conf \
            -I@top_srcdir@/src/xen \
-           @LIBXML_CFLAGS@ \
+           $(LIBXML_CFLAGS) \
 	   -DPROXY  -DLOCALEBASEDIR=\""$(datadir)/locale"\" \
            -DGETTEXT_PACKAGE=\"$(PACKAGE)\" $(WARN_CFLAGS) $(XEN_CFLAGS)