Changed backtrace conditionals to use a feature switch

Changed backtrace conditionals to use a feature switch

[Problem] backtrace(), backtrace_symbols() and demangle are gcc specific features which aren't
present on other platforms.
[Cause]   Works for base linux
[Solution] Ensure that the feature is wrapped with a Dali feature flag in configure. It is
enabled by default (logging the backtrace on exception), but can be disabled at configuration
time by using
 ./configure --disable-backtrace

Change-Id: Ia8032b057489a563d1ba5339ed2cab7687906c1b
Signed-off-by: David Steele <david.steele@partner.samsung.com>