Use a patch.

BUILD.bazel

@@ -209,12 +209,10 @@ cc_library(
     ],
     copts = [
         # Where p4c should look for p4include at runtime.
+        ("-DCONFIG_PKGDATADIR=\\\"external/%s\\\"" % repository_name()),
         # This will work only if the binary is executed by Bazel. For a general
         # solution, we would need to make p4c aware of Bazel, specifically:
         # https://github.com/bazelbuild/bazel/blob/master/tools/cpp/runfiles/runfiles_src.h
-        ("-DCONFIG_PKGDATADIR=\\\"external/%s\\\"" % repository_name()),
-        # Abseil does not export its includes, we add them manually.
-        "-Iexternal/com_google_absl",
     ],
     textual_hdrs = glob([
         "ir/**/*.h",

bazel/abseil.patch

@@ -0,0 +1,15 @@
+diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
+index 0eb735da..5dd4aa67 100644
+--- a/absl/base/BUILD.bazel
++++ b/absl/base/BUILD.bazel
+@@ -137,6 +137,10 @@ cc_library(
+         "policy_checks.h",
+     ],
+     copts = ABSL_DEFAULT_COPTS,
++    # This library is the root of the absl dependency tree.
++    # If we add this `includes` path here, then all absl headers
++    # can be included via angle brackets in external projects.
++    includes = ["../.."],
+     linkopts = ABSL_DEFAULT_LINKOPTS,
+ )
+

bazel/p4c_deps.bzl

@@ -77,6 +77,8 @@ filegroup(
             name = "com_google_absl",
             url = "https://github.com/abseil/abseil-cpp/releases/download/20240116.1/abseil-cpp-20240116.1.tar.gz",
             strip_prefix = "abseil-cpp-20240116.1",
+            patches = ["@com_github_p4lang_p4c//:bazel/abseil.patch"],
+            patch_args = ["-p1"],
             sha256 = "3c743204df78366ad2eaf236d6631d83f6bc928d1705dd0000b872e53b73dc6a",
         )
     if not native.existing_rule("com_google_protobuf"):