Prefer system Abseil if available

Newer versions of Protobuf (22+) pull in Abseil as a dependency. We
want to avoid using our bundled copy for these cases, as this will
likely conflict with the version of Abseil that Protobuf uses.

The simplest way to do this seems to be to just prefer a system
installation of Abseil if it's available.

Signed-off-by: Carlo Cabrera <[email protected]>

CMakeLists.txt

@@ -50,6 +50,13 @@ else()
 endif()
 endif()
 
+find_package(absl CONFIG)
+if(absl_FOUND)
+  MESSAGE(STATUS "System absl found, using")
+else()
+  MESSAGE(STATUS "System absl not found, using bundled version")
+endif()
+
 # Set default build type.
 if(NOT CMAKE_BUILD_TYPE)
   message(STATUS "Setting build type to 'RelWithDebInfo' as none was specified.")
@@ -157,7 +164,9 @@ endif()
 
 include_directories(.)
 include_directories(src)
-include_directories(third_party/abseil-cpp)
+if(NOT absl_FOUND)
+  include_directories(third_party/abseil-cpp)
+endif()
 include_directories("${CMAKE_CURRENT_BINARY_DIR}/src")
 
 # Baseline build flags.
@@ -295,7 +304,9 @@ if(UNIX OR MINGW)
   endif()
 endif()
 
-add_subdirectory(third_party/abseil-cpp)
+if(NOT absl_FOUND)
+  add_subdirectory(third_party/abseil-cpp)
+endif()
 list(APPEND LIBBLOATY_LIBS absl::strings)
 list(APPEND LIBBLOATY_LIBS absl::optional)
 list(APPEND LIBBLOATY_LIBS Threads::Threads)