Use CMP instead of FetchContent for external libraries

ext/CMakeLists.txt

@@ -1,14 +1,53 @@
+message(STATUS "Add CPM.cmake")
+# download CPM.cmake
+file(
+        DOWNLOAD
+        https://github.com/cpm-cmake/CPM.cmake/releases/download/v0.38.6/CPM.cmake
+        ${CMAKE_BINARY_DIR}/cmake/CPM.cmake
+        EXPECTED_HASH SHA256=11c3fa5f1ba14f15d31c2fb63dbc8628ee133d81c8d764caad9a8db9e0bacb07
+)
+include(${CMAKE_BINARY_DIR}/cmake/CPM.cmake)
+
+message(STATUS "Fetching external libraries started")
+
+
 if (NOT EMSCRIPTEN)
-    add_subdirectory(openal-soft)
+    message(STATUS "Fetching openal-soft")
+    CPMAddPackage(
+            NAME openal-soft
+            GITHUB_REPOSITORY kcat/openal-soft
+            GIT_TAG 1.21.1
+            OPTIONS
+            "ALSOFT_UTILS OFF"
+            "ALSOFT_EXAMPLES OFF"
+            "ALSOFT_UPDATE_BUILD_VERSION OFF"
+    )
 endif ()
 
-add_subdirectory(libnyquist)
+message(STATUS "Fetching libnyquist")
+CPMAddPackage(
+        NAME libnyquist
+        GITHUB_REPOSITORY ddiakopoulos/libnyquist
+        GIT_TAG master
+        OPTIONS
+        "LIBNYQUIST_BUILD_EXAMPLE OFF"
+)
 
 if (OALPP_ENABLE_UNIT_TESTS)
-    add_subdirectory(catch2)
+    message(STATUS "Fetching catch2")
+    CPMAddPackage(
+            NAME catch2
+            GITHUB_REPOSITORY catchorg/Catch2
+            GIT_TAG v2.13.8
+    )
 endif ()
 
 
 if (OALPP_ENABLE_APPROVAL_TESTS)
-    add_subdirectory(approvaltestscpp)
+    message(STATUS "Fetching approvaltests")
+    CPMAddPackage(
+            NAME approvaltests
+            GITHUB_REPOSITORY approvals/ApprovalTests.cpp
+            GIT_TAG v.10.12.2
+    )
 endif ()