[CMake] make the headercheck feature configurable

As proposed in FS1584, make headercheck is now disabled by
default in cmake. This fact and how to enable it is printed
upon "make headercheck".

cmake/modules/Headercheck.cmake

 # sets up a global property with the names of all header files
 # in the module and a global target depending on all checks
 macro(setup_headercheck)
-  #glob for headers
-  file(GLOB_RECURSE all_headers "*.hh")
-  # strip hidden files
-  string(REGEX REPLACE "[^;]*/\\.[^;]*\\.hh;?" "" headers "${all_headers}")
-  set_property(GLOBAL PROPERTY headercheck_list ${headers})
+  if(ENABLE_HEADERCHECK)
+    #glob for headers
+    file(GLOB_RECURSE all_headers "*.hh")
+    # strip hidden files
+    string(REGEX REPLACE "[^;]*/\\.[^;]*\\.hh;?" "" headers "${all_headers}")
+    set_property(GLOBAL PROPERTY headercheck_list ${headers})
+  endif()
 
   #define headercheck target
   dune_common_script_dir(SCRIPT_DIR)
-  add_custom_target(headercheck ${CMAKE_COMMAND} -P ${SCRIPT_DIR}/FinalizeHeadercheck.cmake
+  add_custom_target(headercheck ${CMAKE_COMMAND} -P ${SCRIPT_DIR}/FinalizeHeadercheck.cmake -DENABLE_HEADERCHECK=${ENABLE_HEADERCHECK}
                   WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
 endmacro(setup_headercheck)
 
@@ -44,29 +46,31 @@ endmacro(exclude_all_but_from_headercheck)
 
 # configure all headerchecks
 macro(finalize_headercheck)
-  get_property(headerlist GLOBAL PROPERTY headercheck_list)
-  foreach(header ${headerlist})
-    #do some name conversion
-    string(REGEX REPLACE ".*/([^/]*)" "\\1" simple ${header})
-    string(REPLACE ${PROJECT_SOURCE_DIR} "" rel ${header})
-    string(REGEX REPLACE "(.*)/[^/]*" "\\1" relpath ${rel})
-    string(REGEX REPLACE "/" "_" targname ${rel})
+  if(ENABLE_HEADERCHECK)
+    get_property(headerlist GLOBAL PROPERTY headercheck_list)
+    foreach(header ${headerlist})
+      #do some name conversion
+      string(REGEX REPLACE ".*/([^/]*)" "\\1" simple ${header})
+      string(REPLACE ${PROJECT_SOURCE_DIR} "" rel ${header})
+      string(REGEX REPLACE "(.*)/[^/]*" "\\1" relpath ${rel})
+      string(REGEX REPLACE "/" "_" targname ${rel})
 
-    #generate the headercheck .cc file
-    file(WRITE ${CMAKE_BINARY_DIR}/headercheck/${rel}.cc "#ifdef HAVE_CONFIG_H\n#include<config.h>\n#endif\n#include<${simple}>\n#include<${simple}>\nint main(){return 0;}")
+      #generate the headercheck .cc file
+      file(WRITE ${CMAKE_BINARY_DIR}/headercheck/${rel}.cc "#ifdef HAVE_CONFIG_H\n#include<config.h>\n#endif\n#include<${simple}>\n#include<${simple}>\nint main(){return 0;}")
 
-    # add target for the check of current header, this is implemented as a library
-    # to prevent CMake from automatically trying to link the target, functionality
-    # of macro try_compile() is unfortunately not availbale due to it not being scriptable.
-    add_library(headercheck_${targname} STATIC EXCLUDE_FROM_ALL
-      ${CMAKE_BINARY_DIR}/headercheck/${rel}.cc)
-    add_dependencies(headercheck headercheck_${targname})
+      # add target for the check of current header, this is implemented as a library
+      # to prevent CMake from automatically trying to link the target, functionality
+      # of macro try_compile() is unfortunately not availbale due to it not being scriptable.
+      add_library(headercheck_${targname} STATIC EXCLUDE_FROM_ALL
+        ${CMAKE_BINARY_DIR}/headercheck/${rel}.cc)
+      add_dependencies(headercheck headercheck_${targname})
 
-    #add PKG_ALL_FLAGS and the directory where the header is located
-    set_property(TARGET headercheck_${targname}
-      APPEND_STRING PROPERTY COMPILE_FLAGS "-DHEADERCHECK -I${PROJECT_SOURCE_DIR}${relpath} -I${CMAKE_BINARY_DIR}")
-    set_property(TARGET headercheck_${targname} PROPERTY ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/headercheck/${relpath}")
-    add_dune_all_flags(headercheck_${targname})
-    unset(headercheck_${targname}_LIB_DEPENDS CACHE)
-  endforeach(header ${headerlist})
+      #add PKG_ALL_FLAGS and the directory where the header is located
+      set_property(TARGET headercheck_${targname}
+        APPEND_STRING PROPERTY COMPILE_FLAGS "-DHEADERCHECK -I${PROJECT_SOURCE_DIR}${relpath} -I${CMAKE_BINARY_DIR}")
+      set_property(TARGET headercheck_${targname} PROPERTY ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/headercheck/${relpath}")
+      add_dune_all_flags(headercheck_${targname})
+      unset(headercheck_${targname}_LIB_DEPENDS CACHE)
+    endforeach(header ${headerlist})
+  endif()
 endmacro(finalize_headercheck)

cmake/scripts/FinalizeHeadercheck.cmake

 # this is script is called at the end of all header checks
-message("Headerchecks finished! Rerun CMake if a new file has not been checked!")
+if(ENABLE_HEADERCHECK)
+  message("Headerchecks finished! Rerun CMake if a new file has not been checked!")
+else()
+  message("The headercheck feature is currently disabled. You can enable it by adding ENABLE_HEADERCHECK=1 to your cmake flags.")
+endif()
 
 #message("Running make clean on headercheck targets...")
 #this cleans the build directory from pollution through headerchecks but prevents caching... :/