asterisk-11.x and asterisk-13.x: Disable LOW_MEMORY by default

asterisk-11.x and asterisk-13.x: Disable LOW_MEMORY by default

In issue #173 a segmentation fault occuring during a simple SIP call was
reported. Compiling with LOW_MEMORY disabled the reproducible segfault
disappeared.

This commits adds a switch for LOW_MEMORY. Default is off. If anybody
wants to debug the segfault s/he can enable it.

The segfault was reported for Asterisk 11. Disable LOW_MEMORY on
Asterisk 13 anyway, better safe than sorry.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>