Add patch to allow custom package downloads

libs/CMakeLists.txt

 include(FetchContent)
 
 if(ENABLE_NANOFLANN)
-  set(NANOFLANN_BUILD_EXAMPLES OFF CACHE BOOL "Do not build examples for nanoflann" FORCE)
-  set(NANOFLANN_BUILD_TESTS OFF CACHE BOOL "Do not build tests for nanoflann" FORCE)
-  FetchContent_Declare(nanoflann                                # name of the content
-    GIT_REPOSITORY https://github.com/jlblancoc/nanoflann.git   # the repository
-    GIT_TAG        v1.6.2                                       # the tag
-    )
-  FetchContent_GetProperties(nanoflann)
-  if(NOT nanoflann_POPULATED)
-    FetchContent_Populate(nanoflann)
+  if(NOT nanoflann_SOURCE_DIR)
+    set(NANOFLANN_BUILD_EXAMPLES OFF CACHE BOOL "Do not build examples for nanoflann" FORCE)
+    set(NANOFLANN_BUILD_TESTS OFF CACHE BOOL "Do not build tests for nanoflann" FORCE)
+    FetchContent_Declare(nanoflann                                # name of the content
+      GIT_REPOSITORY https://github.com/jlblancoc/nanoflann.git   # the repository
+      GIT_TAG        v1.6.2                                       # the tag
+      )
+    FetchContent_GetProperties(nanoflann)
+    if(NOT nanoflann_POPULATED)
+      FetchContent_Populate(nanoflann)
+    endif()
   endif()
 
   file(GLOB NANOFLANN_HEADERS CONFIGURE_DEPENDS "${nanoflann_SOURCE_DIR}/include/*.hpp")
@@ -25,13 +27,15 @@ if(ENABLE_NANOFLANN)
 endif()
 
 if(ENABLE_ALGLIB)
-  FetchContent_Declare(alglib
-    URL https://www.alglib.net/translator/re/alglib-4.03.0.cpp.gpl.tgz
-    DOWNLOAD_EXTRACT_TIMESTAMP TRUE
-    PATCH_COMMAND
-      ${CMAKE_COMMAND} -E create_symlink <SOURCE_DIR>/src <SOURCE_DIR>/alglib
-  )
-  FetchContent_MakeAvailable(alglib)
+  if(NOT alglib_SOURCE_DIR)
+    FetchContent_Declare(alglib
+      URL https://www.alglib.net/translator/re/alglib-4.03.0.cpp.gpl.tgz
+      DOWNLOAD_EXTRACT_TIMESTAMP TRUE
+      PATCH_COMMAND
+        ${CMAKE_COMMAND} -E create_symlink <SOURCE_DIR>/src <SOURCE_DIR>/alglib
+    )
+    FetchContent_MakeAvailable(alglib)
+  endif()
 
   file(GLOB ALGLIB_HEADERS CONFIGURE_DEPENDS "${alglib_SOURCE_DIR}/src/*.hpp")
   file(GLOB ALGLIB_SOURCES CONFIGURE_DEPENDS "${alglib_SOURCE_DIR}/src/*.cpp")