waf-1.0.tcl: Move --nocache to configure.post_args

This allows Portfiles to use configure.args without needing to remember to
-append to it. Some existing Portfiles already aren't doing that.

And append to configure.post_args, rather than overwriting it, so as to
interoperate with other PortGroups that might also need to add flags here.

_resources/port1.0/group/waf-1.0.tcl

@@ -6,7 +6,7 @@ default waf.python {${prefix}/bin/python2.7}
 depends_build-append    port:python27
 
 configure.cmd           ${waf.python} ./waf configure
-configure.args          --nocache
+configure.post_args-append  --nocache
 
 configure.universal_args-delete --disable-dependency-tracking