Revert "Replace git submodules with CMake FetchContent_Declare"

This reverts commit 05acd97.

.gitmodules

@@ -1,3 +1,9 @@
+[submodule "extlib/tlx"]
+	path = extlib/tlx
+	url = https://github.com/tlx/tlx
+[submodule "extlib/utils"]
+	path = extlib/utils
+	url = https://github.com/pasta-toolbox/utils
 [submodule "extlib/doxygen-awesome-css"]
 	path = extlib/doxygen-awesome-css
 	url = https://github.com/jothepro/doxygen-awesome-css.git

CMakeLists.txt

@@ -18,13 +18,11 @@
 #
 ################################################################################
 
-cmake_minimum_required(VERSION 3.24 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.4.1 FATAL_ERROR)
 
 set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-include(FetchContent)
-
 project(pasta_bit_vector)
 
 # Options when compiling pasta::bit_vector
@@ -38,19 +36,16 @@ option(PASTA_BIT_VECTOR_BUILD_BENCHMARKS
 option(PASTA_BIT_VECTOR_COVERAGE_REPORTING
   "Enable coverage reporting for pasta::bit_vector" OFF)
 
-FetchContent_Declare(
-  tlx
-  GIT_REPOSITORY https://github.com/tlx/tlx.git
-  GIT_TAG        b6af589 #release 0.6.1
-)
-
-FetchContent_Declare(
-  pasta_utils
-  GIT_REPOSITORY https://github.com/pasta-toolbox/utils.git
-  GIT_TAG        92828b8 #main
-)
+# Add tlx subdirectory only, if it has not been added by another project already
+if(NOT TARGET tlx)
+  add_subdirectory(extlib/tlx)
+endif()
 
-FetchContent_MakeAvailable(tlx pasta_utils)
+# Add pasta::utils subdirectory only, if it has not been added by another
+# project already
+if (NOT TARGET pasta_utils)
+  add_subdirectory(extlib/utils)
+endif()
 
 # Optional code coverage (library compile options are only set if coverage
 # reporting is enabled