Debug|Unix|libdeng2: Added a backtrace print macro

Perhaps the Best Thing Ever for non-invasive debugging.

doomsday/libdeng2/include/de/libdeng2.h

@@ -137,6 +137,19 @@
 #  define DENG2_DEBUG
 #  ifdef DENG2_USE_QT
 #    define DENG2_ASSERT(x) Q_ASSERT(x)
+#    ifdef UNIX
+#      include <execinfo.h>
+/**
+ * @macro DENG2_PRINT_BACKTRACE
+ * Debug utility for dumping the current backtrace using qDebug.
+ */
+#      define DENG2_PRINT_BACKTRACE() { \
+    void *callstack[128]; \
+    int i, frames = backtrace(callstack, 128); \
+    char** strs = backtrace_symbols(callstack, frames); \
+    for(i = 0; i < frames; ++i) qDebug("%s", strs[i]); \
+    free(strs); }
+#    endif
 #  else
 #    define DENG2_ASSERT(x) assert(x)
 #  endif