update submodule url and init check

* use absolute path for submodule
* use CMakeLists.txt to check whether submodule is already initialized

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "extern/ENCRYPTO_utils"]
 	path = extern/ENCRYPTO_utils
-	url = ../ENCRYPTO_utils.git
+	url = https://github.com/encryptogroup/ENCRYPTO_utils.git
 [submodule "extern/OTExtension"]
 	path = extern/OTExtension
-	url = ../OTExtension.git
+	url = https://github.com/encryptogroup/OTExtension.git

CMakeLists.txt

@@ -29,7 +29,7 @@ if(ENCRYPTO_utils_FOUND)
     message(STATUS "Found ENCRYPTO_utils")
 elseif(NOT ENCRYPTO_utils_FOUND AND NOT TARGET ENCRYPTO_utils::encrypto_utils)
     message("ENCRYPTO_utils was not found: add ENCRYPTO_utils subdirectory")
-    if(NOT EXISTS "${PROJECT_SOURCE_DIR}/extern/ENCRYPTO_utils/.git")
+    if(NOT EXISTS "${PROJECT_SOURCE_DIR}/extern/ENCRYPTO_utils/CMakeLists.txt")
         find_package(Git REQUIRED)
         message("initialize Git submodule: extern/ENCRYPTO_utils")
         execute_process(COMMAND git submodule update --init extern/ENCRYPTO_utils
@@ -43,7 +43,7 @@ if(OTExtension_FOUND)
     message(STATUS "Found OTExtension")
 elseif (NOT OTExtension_FOUND AND NOT TARGET OTExtension::otextension)
     message("OTExtension was not found: add OTExtension subdirectory")
-    if(NOT EXISTS "${PROJECT_SOURCE_DIR}/extern/OTExtension/.git")
+    if(NOT EXISTS "${PROJECT_SOURCE_DIR}/extern/OTExtension/CMakeLists.txt")
         find_package(Git REQUIRED)
         message("initialize Git submodule: extern/OTExtension")
         execute_process(COMMAND git submodule update --init extern/OTExtension