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ARROW-4197: [C++] Better Emscripten support

A few changes for better compatibility with the Emscripten compiler for WebAssembly

- expose the `-ggdb` flag as an option (unsupported by emscripten)
- the `-undefined dynamic_lookup` flag should be set on apple, but not when using emscripten
- allow for `backtrace` to be turned off even if found (no `execinfo.h` available, from `util/logging.cc`)

Author: Tim Paine <t.paine154@gmail.com>

Closes #3350 from timkpaine/emscripten and squashes the following commits:

e3661ff52 <Tim Paine> restore default ggdb behavior, use better environment variable to detect emscripten and add a comment explaining this
a0e91a77c <Tim Paine> expose backtrace at top level, re-add -g, make backtrace private scope again
b8f0c8068 <Tim Paine> Merge branch 'master' into emscripten
5308f6b49 <Tim Paine> fix for emscripten