paho.mqtt.cpp: set required C++11 standard

net/paho.mqtt.cpp/Portfile

@@ -24,9 +24,15 @@ depends_lib-append \
                     port:paho.mqtt.c
 
 # We enable SSL so that the paho.mqtt.cpp library builds without issues
+# TODO: enable static libs as well once this is fixed:
+# https://github.com/eclipse/paho.mqtt.cpp/issues/515
 configure.args-append \
+                    -DPAHO_BUILD_SHARED=ON \
                     -DPAHO_WITH_SSL=ON
 
 checksums           rmd160  543046a3d39cb2bb35266354394130520273b0e6 \
                     sha256  9c7c8f95960e669465390de2d6d65e30248ad8d30895f52c71ba086b6a41a61a \
                     size    235708
+
+# Target "paho-mqttpp3" requires the language dialect "CXX11"
+compiler.cxx_standard   2011