Add an option to check the syntax of headers, individually.

That feature can be enabled with g++, clang++, and icpc (intel). It could
be implemented for MSVC, with the flag /Zs, but that is not yet done (and
probably will not).

The option is controlled by a CMake option, CGAL_ENABLE_CHECK_HEADERS, that
is disabled by default. If that option is enabled by the user, then CMake
will check if the compiler $CXX understand the syntax:
  $CXX -x c++ -fsyntax-only CGAL/header.h
and send an error otherwise.

Then phony targets will be created:
  - a target check_CGAL__header_h for each header <CGAL/header.h>,
  - a target check_pkg_<pkg> for each package <pgk>,
  - and a target check_headers for the whole CGAL.

Those new targets currently give a lot of compilation errors if
CGAL_ENABLE_CHECK_HEADERS is enabled!

+ fix several missing includes in Mesh_2.

Installation/CMakeLists.txt

@@ -948,3 +948,63 @@ if( WITH_CPACK AND EXISTS "${CMAKE_ROOT}/Modules/CPack.cmake" )
     
 endif()
 
+
+if ( CGAL_BRANCH_BUILD )
+  option(CGAL_ENABLE_CHECK_HEADERS 
+    "Enable the special targets \"check_pkg_headers\", and \"check_pkg_<package>_headers\" for each package"
+    FALSE)
+
+  if(CGAL_ENABLE_CHECK_HEADERS)
+
+    if(NOT DEFINED CGAL_CHECK_SYNTAX_ONLY)
+      execute_process(COMMAND 
+        ${CMAKE_CXX_COMPILER} -x c++ -fsyntax-only ${CMAKE_CURRENT_SOURCE_DIR}/config/support/test_syntaxonly.cpp
+        ERROR_QUIET
+        RESULT_VARIABLE ok)
+      if(ok EQUAL 0)
+        set(CGAL_CHECK_SYNTAX_ONLY ON CACHE INTERNAL "")
+      else()
+        set(CGAL_CHECK_SYNTAX_ONLY OFF CACHE INTERNAL "")
+      endif()
+    endif(NOT DEFINED CGAL_CHECK_SYNTAX_ONLY)
+
+    if(NOT CGAL_CHECK_SYNTAX_ONLY)
+      message(FATAL_ERROR "Your compiler does not seem to support -fsyntax-only.
+You must disable CGAL_ENABLE_CHECK_HEADERS.")
+    endif()
+
+    ## Fill the variable include_options with all the -I and -isystem options
+    set(include_options)
+    foreach (incdir ${CGAL_INCLUDE_DIRS})
+      list(APPEND include_options "-I${incdir}")
+    endforeach()
+    foreach (incdir ${CGAL_3RD_PARTY_INCLUDE_DIRS})
+      list(APPEND include_options "-isystem${incdir}")
+    endforeach()
+    include_directories ( SYSTEM ${CGAL_3RD_PARTY_INCLUDE_DIRS} )
+
+    ## Loop on package and headers
+    set(check_pkg_target_list)
+    foreach (package ${CGAL_CONFIGURED_PACKAGES_NAMES})
+      if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/../${package}/include)
+        set(depends "")
+        file(GLOB_RECURSE ${package}_HEADERS 
+          RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}/../${package}/include" 
+          "${CMAKE_CURRENT_SOURCE_DIR}/../${package}/include/*.h")
+        foreach(header ${${package}_HEADERS})
+          string(REPLACE "/" "__" header2 "${header}")
+          string(REPLACE "." "_" header2 "${header2}")
+          add_custom_target(check_${header2}
+            COMMAND ${CMAKE_CXX_COMPILER} ${CGAL_3RD_PARTY_DEFINITIONS} ${CGAL_DEFINITIONS} ${include_options} -x c++ -fsyntax-only "${CMAKE_CURRENT_SOURCE_DIR}/../${package}/include/${header}"
+            VERBATIM
+            COMMENT "Check header ${header}"
+            )
+          list(APPEND depends check_${header2})
+        endforeach() # look on headers
+        add_custom_target(check_pkg_${package}_headers DEPENDS ${depends})
+        list(APPEND check_pkg_target_list check_pkg_${package}_headers)
+      endif() # if the package has an include directory
+    endforeach() # loop on packages
+    add_custom_target(check_headers DEPENDS ${check_pkg_target_list})
+  endif()
+endif( CGAL_BRANCH_BUILD )

Mesh_2/include/CGAL/Mesh_2/Refine_edges.h

@@ -24,6 +24,8 @@
 #include <CGAL/Mesher_level.h>
 #include <CGAL/Meshes/Triangulation_mesher_level_traits_2.h>
 #include <CGAL/Meshes/Filtered_queue_container.h>
+#include <CGAL/tags.h>
+#include <CGAL/assertions.h>
 
 #include <utility>
 #include <iterator>

Mesh_2/include/CGAL/Mesher_level.h

@@ -21,6 +21,8 @@
 #ifndef CGAL_MESHER_LEVEL_H
 #define CGAL_MESHER_LEVEL_H
 
+#include <string>
+
 namespace CGAL {
 
 enum Mesher_level_conflict_status {

Mesh_2/include/CGAL/Meshes/Triangulation_mesher_level_traits_3.h

@@ -24,6 +24,7 @@
 #include <vector>
 #include <CGAL/Mesher_level.h>
 #include <CGAL/Mesher_level_default_implementations.h>
+#include <CGAL/tags.h>
 
 namespace CGAL {