cmake: Remove SHELL: tag in process_flags function

When fetching include, system include, compiler options, or compiler
defines, then it is important to strip the SHELL: (no de-duplication)
tag of any flag.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
diff --git a/cmake/extensions.cmake b/cmake/extensions.cmake
index a08d251..95ce0bb 100644
--- a/cmake/extensions.cmake
+++ b/cmake/extensions.cmake
@@ -293,6 +293,11 @@
     endforeach()
 
     if(NOT is_compile_lang_generator_expression)
+      # SHELL is used to avoid de-deplucation, but when process flags
+      # then this tag must be removed to return real compile/linker flags.
+      if(flag MATCHES "SHELL:[ ]*(.*)")
+        separate_arguments(flag UNIX_COMMAND ${CMAKE_MATCH_1})
+      endif()
       list(APPEND tmp_list ${flag})
     endif()
   endforeach()