Not every __sun has backtrace().

I don't know how to test for Open Solaris that has support for
backtrace() so for now removing the #ifdef that breaks compilation under
other Solaris flavors.

src/config.h

@@ -56,7 +56,7 @@
 #endif
 
 /* Test for backtrace() */
-#if defined(__APPLE__) || defined(__linux__) || defined(__sun)
+#if defined(__APPLE__) || defined(__linux__)
 #define HAVE_BACKTRACE 1
 #endif