Re-enable backtraces in glog

Autodetect doesn't work, need to hard-code it in.

Change-Id: I1bc9f4860605439ce5cb301e70233b655478efb8
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/third_party/google-glog/bazel/glog.bzl b/third_party/google-glog/bazel/glog.bzl
index 2e7a8d4..b4825d3 100644
--- a/third_party/google-glog/bazel/glog.bzl
+++ b/third_party/google-glog/bazel/glog.bzl
@@ -95,6 +95,8 @@
         "-DHAVE_SYS_SYSCALL_H",
         # For src/logging.cc to create symlinks.
         "-DHAVE_UNISTD_H",
+        "-DHAVE_EXECINFO_H",
+        "-DHAVE_EXECINFO_BACKTRACE",
         "-fvisibility-inlines-hidden",
         "-fvisibility=hidden",
     ]