Set SIMPL_USE_GHC_FILESYSTEM=ON if CMAKE_OSX_DEPLOYMENT_TARGET < 10.15

Signed-off-by: Michael Jackson <mike.jackson@bluequartz.net>

Author: imikejackson

CMakeLists.txt

@@ -431,6 +431,10 @@ if(APPLE)
   endif()
   message(STATUS "SIMPL: DARWIN_VERSION ${DARWIN_VERSION}")
   message(STATUS "SIMPL: CMAKE_OSX_DEPLOYMENT_TARGET: ${CMAKE_OSX_DEPLOYMENT_TARGET}")
+  if(CMAKE_OSX_DEPLOYMENT_TARGET VERSION_LESS 10.15)
+    message(STATUS "SIMPL: CMAKE_OSX_DEPLOYMENT_TARGET less than 10.15. Enabling SIMPL_USE_GHC_FILESYSTEM")
+    set(SIMPL_USE_GHC_FILESYSTEM ON)
+  endif()
 endif()
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux" AND ${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9.0)
   set(SIMPL_USE_GHC_FILESYSTEM ON)