Use the Abseil flags library when Abseil is present

When built with `--define=absl=1` under Bazel, GoogleTest
flags use ABSL_FLAG instead of GoogleTest's own implementation.

There are some minor behavior differences in this mode.

The most notable difference is that unrecognized flags result
in a flag parsing error, and are not returned to the user though
a modified argc/argv, unless they appear after the positional
argument delimiter ("--").

For example, to pass a non-Abseil flag, you would have to do
./mytest --gtest_color=false -- --myflag=myvalue

The documentation at https://abseil.io/docs/cpp/guides/flags
may be helpful in understanding the behavior.

There are some other minor differences. For example,
passing --help results in the program returning 1 instead of 0.

https://github.com/google/googletest/issues/3646

PiperOrigin-RevId: 439312700
Change-Id: Id696a25f50f24a5b1785c45ca8fa59794f86fd5c

BUILD.bazel

@@ -120,8 +120,14 @@ cc_library(
     linkopts = select({
         ":qnx": ["-lregex"],
         ":windows": [],
-        ":freebsd": ["-lm", "-pthread"],
-        ":openbsd": ["-lm", "-pthread"],
+        ":freebsd": [
+            "-lm",
+            "-pthread",
+        ],
+        ":openbsd": [
+            "-lm",
+            "-pthread",
+        ],
         "//conditions:default": ["-pthread"],
     }),
     deps = select({
@@ -129,6 +135,10 @@ cc_library(
             "@com_google_absl//absl/debugging:failure_signal_handler",
             "@com_google_absl//absl/debugging:stacktrace",
             "@com_google_absl//absl/debugging:symbolize",
+            "@com_google_absl//absl/flags:flag",
+            "@com_google_absl//absl/flags:parse",
+            "@com_google_absl//absl/flags:reflection",
+            "@com_google_absl//absl/flags:usage",
             "@com_google_absl//absl/strings",
             "@com_google_absl//absl/types:any",
             "@com_google_absl//absl/types:optional",