MINOR: abort() on my_unreachable() when DEBUG_USE_ABORT is set.

Hopefully this helps static analysis tools detecting that the code after that
call is unreachable.

See GitHub Issue #1075.
diff --git a/include/haproxy/compiler.h b/include/haproxy/compiler.h
index e5fae3e..fba6dc3 100644
--- a/include/haproxy/compiler.h
+++ b/include/haproxy/compiler.h
@@ -66,11 +66,15 @@
  * above which can more aggressively detect null dereferences. The builtin
  * below was introduced in gcc 4.5, and before it we didn't care.
  */
+#ifdef DEBUG_USE_ABORT
+#define my_unreachable() abort()
+#else
 #if __GNUC__ >= 5 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 5)
 #define my_unreachable() __builtin_unreachable()
 #else
 #define my_unreachable()
 #endif
+#endif
 
 /* This macro may be used to block constant propagation that lets the compiler
  * detect a possible NULL dereference on a variable resulting from an explicit