Added boost_serialization to Portfile

This allows port "plumed +allmodules" to use DRR module.

macports/Portfile.in

@@ -74,8 +74,11 @@ configure.ldflags-append -lmatheval -lxdrfile -lz -lgsl
 depends_lib-append port:libmatheval port:xdrfile port:zlib port:gsl
 
 # This variant enables optional modules in PLUMED.
+# Notice that one of the optional modules (drr) requires boost_serialization
 variant allmodules description {Enable all optional modules} {
-  configure.args-append --enable-modules=all
+  configure.args-append --enable-modules=all --enable-boost-serialization
+  configure.ldflags-append -lboost_serialization
+  depends_lib-append port:boost
 }
 
 # Link lapack/blas libraries