Disable backtrace support in AIX (danmar#7986)

PR  - danmar#7773 breaks AIX build. 
In AIX, execinfo.h header file, backtrace and backtrace_symbols APIs are
not present. So we need to disable backtrace support for AIX.

Author: reshmavk

lib/config.h

@@ -208,7 +208,7 @@
 #define USE_WINDOWS_SEH
 #endif
 
-#if !defined(NO_UNIX_BACKTRACE_SUPPORT) && defined(__GNUC__) && !defined(__APPLE__) && !defined(__CYGWIN__) && !defined(__MINGW32__) && !defined(__NetBSD__) && !defined(__OpenBSD__) && !defined(__SVR4) && !defined(__QNX__)
+#if !defined(NO_UNIX_BACKTRACE_SUPPORT) && defined(__GNUC__) && !defined(__APPLE__) && !defined(__CYGWIN__) && !defined(__MINGW32__) && !defined(__NetBSD__) && !defined(__OpenBSD__) && !defined(__SVR4) && !defined(__QNX__) && !defined(_AIX)
 #define USE_UNIX_BACKTRACE_SUPPORT
 #endif