Default GTEST_HAS_ABSL=1 since abseil is an unconditional dep

abseil-cpp is loaded unconditionally under bzlmod (via bazel_dep) and
under WORKSPACE (via googletest_deps()), but GTEST_HAS_ABSL was only
defined when the user passed --define absl=1. As a result, features
gated on the macro (AbslStringify support, absl::flags parsing, the
failure-signal handler) silently stayed off even though the abseil
dependency was being pulled in.

Flip the default: enable absl integration by default and let users
opt out with --define absl=0. Existing scripts passing --define absl=1
keep working as a no-op. Also extract the absl deps list into a local
variable to avoid duplication.

Fixes #4953

BUILD.bazel

@@ -73,10 +73,23 @@ config_setting(
 )
 
 config_setting(
-    name = "has_absl",
-    values = {"define": "absl=1"},
+    name = "absl_disabled",
+    values = {"define": "absl=0"},
 )
 
+_ABSL_DEPS = [
+    "@abseil-cpp//absl/container:flat_hash_set",
+    "@abseil-cpp//absl/debugging:failure_signal_handler",
+    "@abseil-cpp//absl/debugging:stacktrace",
+    "@abseil-cpp//absl/debugging:symbolize",
+    "@abseil-cpp//absl/flags:flag",
+    "@abseil-cpp//absl/flags:parse",
+    "@abseil-cpp//absl/flags:reflection",
+    "@abseil-cpp//absl/flags:usage",
+    "@abseil-cpp//absl/strings",
+    "@re2",
+]
+
 # Library that defines the FRIEND_TEST macro.
 cc_library(
     name = "gtest_prod",
@@ -116,8 +129,8 @@ cc_library(
         "//conditions:default": ["-pthread"],
     }),
     defines = select({
-        ":has_absl": ["GTEST_HAS_ABSL=1"],
-        "//conditions:default": [],
+        ":absl_disabled": [],
+        "//conditions:default": ["GTEST_HAS_ABSL=1"],
     }),
     features = select({
         ":windows": ["windows_export_all_symbols"],
@@ -143,19 +156,8 @@ cc_library(
         "//conditions:default": ["-pthread"],
     }),
     deps = select({
-        ":has_absl": [
-            "@abseil-cpp//absl/container:flat_hash_set",
-            "@abseil-cpp//absl/debugging:failure_signal_handler",
-            "@abseil-cpp//absl/debugging:stacktrace",
-            "@abseil-cpp//absl/debugging:symbolize",
-            "@abseil-cpp//absl/flags:flag",
-            "@abseil-cpp//absl/flags:parse",
-            "@abseil-cpp//absl/flags:reflection",
-            "@abseil-cpp//absl/flags:usage",
-            "@abseil-cpp//absl/strings",
-            "@re2",
-        ],
-        "//conditions:default": [],
+        ":absl_disabled": [],
+        "//conditions:default": _ABSL_DEPS,
     }) + select({
         # `gtest-death-test.cc` has `EXPECT_DEATH` that spawns a process,
         # expects it to crash and inspects its logs with the given matcher,