Adds stacktrace support from Abseil to Google Test

This change adds the ability to generate stacktraces in Google Test on
both failures of assertions/expectations and on crashes. The
stacktrace support is conditionally available only when using Abseil
with Google Test.

To use this support, run the test under Bazel with a command like this:

bazel test --define absl=1 --test_env=GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1 //path/to/your:test

The "--define absl=1" part enables stacktraces on assertion/expectation
failures.

The "--test_env=GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" part enables
the signal handler that logs a stacktrace in the event of a crash
(this also requires the "--define absl=1" part). This is not the
default since it may interfere with existing tests.

BUILD.bazel

@@ -38,7 +38,7 @@ licenses(["notice"])
 
 config_setting(
     name = "windows",
-    values = { "cpu": "x64_windows" },
+    values = {"cpu": "x64_windows"},
 )
 
 config_setting(
@@ -51,7 +51,6 @@ config_setting(
     values = {"define": "absl=1"},
 )
 
-
 # Google Test including Google Mock
 cc_library(
     name = "gtest",
@@ -70,7 +69,7 @@ cc_library(
             "googlemock/src/gmock_main.cc",
         ],
     ),
-    hdrs =glob([
+    hdrs = glob([
         "googletest/include/gtest/*.h",
         "googlemock/include/gmock/*.h",
     ]),
@@ -81,6 +80,14 @@ cc_library(
             "//conditions:default": ["-pthread"],
         },
     ),
+    defines = select(
+        {
+            ":has_absl": [
+                "GTEST_HAS_ABSL=1",
+            ],
+            "//conditions:default": [],
+        },
+    ),
     includes = [
         "googlemock",
         "googlemock/include",
@@ -94,21 +101,18 @@ cc_library(
             "-pthread",
         ],
     }),
-    defines = select ({
-        ":has_absl": [
-        "GTEST_HAS_ABSL=1",
-        ],
-        "//conditions:default": [],
-    }
+    deps = select(
+        {
+            ":has_absl": [
+                "@com_google_absl//absl/debugging:failure_signal_handler",
+                "@com_google_absl//absl/debugging:stacktrace",
+                "@com_google_absl//absl/debugging:symbolize",
+                "@com_google_absl//absl/strings",
+                "@com_google_absl//absl/types:optional",
+            ],
+            "//conditions:default": [],
+        },
     ),
-    deps = select ({
-        ":has_absl": [
-        "@com_google_absl//absl/types:optional",
-        "@com_google_absl//absl/strings"
-        ],
-        "//conditions:default": [],
-    }
-    )
 )
 
 cc_library(