Unbundling abseil
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -100,7 +100,6 @@
   libgav1_dump_options()
 endif()
 
-set(libgav1_abseil_build "${libgav1_build}/abseil")
 set(libgav1_gtest_build "${libgav1_build}/gtest")
 
 # Compiler/linker flags must be lists, but come in from the environment as
@@ -115,18 +114,8 @@
 # Set test-only flags based on LIBGAV1_CXX_FLAGS.
 libgav1_set_test_flags()
 
-set(libgav1_abseil "${libgav1_root}/third_party/abseil-cpp")
-if(NOT EXISTS "${libgav1_abseil}")
-  message(
-    FATAL_ERROR
-      "Abseil not found. This dependency is required by the"
-      " examples & tests and libgav1 when LIBGAV1_THREADPOOL_USE_STD_MUTEX is"
-      " not defined. To continue, download the Abseil repository to"
-      " third_party/abseil-cpp:\n  git \\\n    -C ${libgav1_root} \\\n"
-      "    clone \\\n"
-      "    https://github.com/abseil/abseil-cpp.git third_party/abseil-cpp")
-endif()
-add_subdirectory("${libgav1_abseil}" "${libgav1_abseil_build}" EXCLUDE_FROM_ALL)
+set(absl "/usr/lib/*/cmake/absl")
+find_package(absl REQUIRED)
 
 libgav1_reset_target_lists()
 libgav1_add_dsp_targets()