[master] added CONFIG_H_WAS_INCLUDED (#3812)

ChangeLog

+928.	[build]		fdupont
+	A CONFIG_H_WAS_INCLUDED define has been added to provide
+	a way in source files to check whether config.h has been included.
+	(Trac #3812, git xxx)
+
 927.	[bug]		tmark
 	DHCPv4 no longer attempts to update the lease database with the 
 	generated FQDN when processing DHCPDISCOVERs.

configure.ac

@@ -1387,6 +1387,8 @@ AC_TRY_LINK(
 AM_CONDITIONAL(HAVE_OPTRESET, test "x$var_optreset_exists" != "xno")
 AM_COND_IF([HAVE_OPTRESET], [AC_DEFINE([HAVE_OPTRESET], [1], [Check for optreset?])])
 
+AC_DEFINE([CONFIG_H_WAS_INCLUDED], [1], [config.h inclusion marker])
+
 AC_CONFIG_FILES([compatcheck/Makefile
                  dns++.pc
                  doc/design/datasrc/Makefile