Use system spdlog, cxxopts and gtest instead of the bundled
third_party/ copies. The remaining bundled deps (cpu_features, ruy,
cpuinfo) are not packaged in Alpine.

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -105,7 +105,7 @@ if(MSVC)
 endif()

 find_package(Threads)
-add_subdirectory(third_party/spdlog EXCLUDE_FROM_ALL)
+find_package(spdlog REQUIRED)

 set(PRIVATE_INCLUDE_DIRECTORIES
   ${CMAKE_CURRENT_SOURCE_DIR}/src
--- a/cli/CMakeLists.txt
+++ b/cli/CMakeLists.txt
@@ -1,15 +1,6 @@
-if (NOT IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/../third_party/cxxopts/include")
-  message(FATAL_ERROR "The client dependency repository (cxxopts) is missing! "
-    "You probably didn't clone the project with --recursive. You can include it "
-    "by calling \"git submodule update --init --recursive\"")
-endif()
-
 add_executable(translator
   translator.cc
   )
-target_include_directories(translator
-  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../third_party/cxxopts/include
-  )
 target_link_libraries(translator
   PRIVATE ${PROJECT_NAME}
 )
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,8 +1,6 @@
 enable_testing()

-option(BUILD_GMOCK "" OFF)
-option(INSTALL_GTEST "" OFF)
-add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/../third_party/googletest ${CMAKE_CURRENT_BINARY_DIR}/googletest)
+find_package(GTest REQUIRED)

 add_executable(ctranslate2_test
   attention_test.cc
@@ -21,7 +19,7 @@ target_include_directories(ctranslate2_test PRIVATE
   )
 target_link_libraries(ctranslate2_test PRIVATE
   ${PROJECT_NAME}
-  gtest_main
+  GTest::gtest_main
   )

 add_executable(benchmark_ops