don't pool headers into one target

The idea was to speed up the build process by removing several
unneccessary targets and stage all headers in one go. However,
for some reason, CMake does not think that's a good idea. Only
parts of the headers get staged

cmake_support/OST.cmake

@@ -69,7 +69,6 @@ endmacro()
 # content changed.
 #-------------------------------------------------------------------------------
 macro(copy_if_different FROM_DIR TO_DIR FILES TARGETS TARGET)
-  set(ADD_TARGETS "")
   foreach(SRC ${FILES})
       set(SRCFILE ${SRC})
       if("${FROM_DIR}" STREQUAL "" OR "${FROM_DIR}" STREQUAL "./")
@@ -87,13 +86,8 @@ macro(copy_if_different FROM_DIR TO_DIR FILES TARGETS TARGET)
       add_custom_command(TARGET "${TARGET}" PRE_BUILD
           DEPENDS ${FROM}
           COMMAND ${CMAKE_COMMAND} -E copy_if_different ${FROM} ${TO}
-          COMMENT ""
-          )
-      list(APPEND ADD_TARGETS ${TO})
+          COMMENT "Staging ${SRC}")
   endforeach()
-  if (TARGETS)
-    set(${TARGETS} ${ADD_TARGETS})
-  endif()
 endmacro()
 
 #-------------------------------------------------------------------------------
@@ -109,20 +103,16 @@ macro(stage_headers HEADERS HEADER_INSTALL_DIR TARGET SUB)
   # building the library
   string(REPLACE "/" "_" _SUB_NO_SLASH "${SUB}")
   string(REPLACE "${PREFIX}_" "" _TARGET "${TARGET}")
-  #message("target before: ${TARGET} after: ${_TARGET}")
   set(_TARGET_NAME ${_TARGET}_${_SUB_NO_SLASH}_headers)
   set(_SUB ${SUB})
   if (NOT _SUB)
     set(_TARGET_NAME ${_TARGET}_headers)
   endif()
-  if (NOT TARGET headers)
-    add_custom_target("headers" COMMENT "")
-  endif()
   set(HEADER_DIR "${HEADER_STAGE_PATH}/${HEADER_INSTALL_DIR}")
   copy_if_different("" "${HEADER_DIR}"
                     "${_ABS_HEADER_NAMES}" ""
-                    "${TARGET}")
-  add_dependencies(${TARGET} "headers")
+                    "${_TARGET_NAME}")
+  add_dependencies(${TARGET} ${_TARGET_NAME})
 endmacro()
 
 #-------------------------------------------------------------------------------