include cmake rules for third party libraries only if it needed

CMakeLists.txt

@@ -77,13 +77,9 @@ include(cmake/init_target.cmake)
 include(cmake/generate_target.cmake)
 include(cmake/target_yasm_sources.cmake)
 include(cmake/external.cmake)
-include(cmake/libabsl.cmake)
-include(cmake/libopenh264.cmake)
 include(cmake/libpffft.cmake)
 include(cmake/librnnoise.cmake)
 include(cmake/libsrtp.cmake)
-include(cmake/libusrsctp.cmake)
-include(cmake/libvpx.cmake)
 include(cmake/libyuv.cmake)
 if (APPLE)
     include(cmake/libsdkmacos.cmake)

cmake/external.cmake

@@ -123,6 +123,9 @@ function(link_libabsl target_name)
         endif()
     endif()
     if (NOT absl_FOUND)
+        if (NOT TARGET libabsl)
+            include(cmake/libabsl.cmake)
+        endif()
         target_link_libraries(${target_name} PRIVATE tg_owt::libabsl)
     endif()
 endfunction()
@@ -139,6 +142,9 @@ function(link_libopenh264 target_name)
         endif()
     endif()
     if (NOT LIBOPENH264_FOUND)
+        if (NOT TARGET libopenh264)
+            include(cmake/libopenh264.cmake)
+        endif()
         target_link_libraries(${target_name} PRIVATE tg_owt::libopenh264)
         target_include_directories(${target_name} PRIVATE ${libopenh264_loc}/include)
     endif()
@@ -156,6 +162,9 @@ function(link_libusrsctp target_name)
         endif()
     endif()
     if (NOT LIBUSRSCTP_FOUND)
+        if (NOT TARGET libusrsctp)
+            include(cmake/libusrsctp.cmake)
+        endif()
         target_link_libraries(${target_name} PRIVATE tg_owt::libusrsctp)
     endif()
 endfunction()
@@ -172,6 +181,9 @@ function(link_libvpx target_name)
         endif()
     endif()
     if (NOT LIBVPX_FOUND)
+        if (NOT TARGET libvpx)
+            include(cmake/libvpx.cmake)
+        endif()
         target_link_libraries(${target_name} PRIVATE tg_owt::libvpx)
         if (is_x86 OR is_x64)
             target_link_libraries(${target_name}