cmake: fix ENABLE_ASAN detection

cmake: fix ENABLE_ASAN detection

Do not add -fsanitize=undefined when ASAN is requested, UBSAN is a
different feature (which could be added later as desired). This makes
the -DENABLE_ASAN=1 option match the autotools --enable-asan option.

Fail hard if ASAN support is requested but not supported, this avoids
surprises when something is wrong. Fix ASAN detection by setting the
linker option too.

Note: if you have previously set ENABLE_ASAN=1 with the broken ASAN
detection, you have to clear your CMakeCache.txt file to redo the
detection.

Change-Id: Iba6ca0da0336eccedd0cf31a251baad9d1aff5b4
Reviewed-on: https://code.wireshark.org/review/15908
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>