Fix build warnings in libevent

Fix warnings related to EV_SSIZE_MAX by correctly defining
_EVENT_SIZEOF_SIZE_T, which is the basis for EV_SSIZE_MAX.

This apparently was only broken for 32 bit host builds of libevent.

Apply -Werror to make future warnings fatal.

Bug: 27075012
Test: mmma -j29 external/libevent for aosp_arm-eng and aosp_x86_64-eng
      speculatively fixed darwin warnings.

Change-Id: I09f804ccefdab15bb77f82b81cb0cb4132b751c8