allow gtest in a sibling directory for application builds

to facility build servers where we cannot use the network

cmake/Scripts/IFEMTesting.cmake

@@ -95,6 +95,11 @@ if(IFEM_INTREE_BUILD)
   include_directories(${IFEM_PATH}/3rdparty/gtest/include)
 elseif(NOT IFEM_AS_SUBMODULE AND NOT IFEM_LIBRARY_BUILD
        AND NOT TARGET gtest)
-  include(DownloadGTest)
-  include_directories(${GTEST_INCLUDE_DIRS})
+  if (EXISTS ${PROJECT_SOURCE_DIR}/../gtest)
+    add_subdirectory(${PROJECT_SOURCE_DIR}/../gtest gtest)
+    include_directories(${PROJECT_SOURCE_DIR}/../gtest/include)
+  else()
+    include(DownloadGTest)
+    include_directories(${GTEST_INCLUDE_DIRS})
+  endif()
 endif()