SystemInformation: Include backtrace-related headers on FreeBSD

This was probably broken for a long while, but the problem was not apparent
because the check for execinfo.h would fail by default because
-I/usr/local/include was not being passed to the compiler when making the
checks for the header's existence.

Now that very recent FreeBSD versions (ie. 10-CURRENT) have NetBSD's
libexecinfo in base (and it is thus installed into /usr), the
backtrace-related checks would pass, but the required headers were not being
included in SystemInformation.cxx.

Change-Id: I3b91ed7ac0e6878035aee202b3336c536cc6d2ff

SystemInformation.cxx

@@ -88,6 +88,15 @@ typedef int siginfo_t;
 #  include <ifaddrs.h>
 #  define KWSYS_SYSTEMINFORMATION_IMPLEMENT_FQDN
 # endif
+# if defined(KWSYS_SYSTEMINFORMATION_HAS_BACKTRACE)
+#  include <execinfo.h>
+#  if defined(KWSYS_SYSTEMINFORMATION_HAS_CPP_DEMANGLE)
+#    include <cxxabi.h>
+#  endif
+#  if defined(KWSYS_SYSTEMINFORMATION_HAS_SYMBOL_LOOKUP)
+#    include <dlfcn.h>
+#  endif
+# endif
 #endif
 
 #if defined(__OpenBSD__) || defined(__NetBSD__)