Provide dependency file flags to generator

Causes compiler modules (currently only GNU) to set a
CMAKE_DEPFILE_FLAGS_${lang} variable, which communicates to
the generator the flags required to cause the compiler to create
dependency files.

Modules/Compiler/GNU.cmake

@@ -24,6 +24,15 @@ macro(__compiler_gnu lang)
   set(CMAKE_SHARED_LIBRARY_${lang}_FLAGS "-fPIC")
   set(CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS "-shared")
 
+  # Older versions of gcc (< 4.5) contain a bug causing them to report a missing
+  # header file as a warning if depfiles are enabled, causing check_header_file
+  # tests to always succeed.  Work around this by disabling dependency tracking
+  # in try_compile mode.
+  GET_PROPERTY(_IN_TC GLOBAL PROPERTY IN_TRY_COMPILE)
+  if(NOT _IN_TC OR CMAKE_FORCE_DEPFILES)
+    set(CMAKE_DEPFILE_FLAGS_${lang} "-MMD -MF <DEPFILE>")
+  endif()
+
   # Initial configuration flags.
   set(CMAKE_${lang}_FLAGS_INIT "")
   set(CMAKE_${lang}_FLAGS_DEBUG_INIT "-g")

Tests/BuildDepends/CMakeLists.txt

@@ -40,6 +40,8 @@ if("${CMAKE_GENERATOR}" MATCHES "Make")
 endif()
 list(APPEND _cmake_options "-DTEST_LINK_DEPENDS=${TEST_LINK_DEPENDS}")
 
+list(APPEND _cmake_options "-DCMAKE_FORCE_DEPFILES=1")
+
 file(MAKE_DIRECTORY ${BuildDepends_BINARY_DIR}/Project)
 message("Creating Project/foo.cxx")
 write_file(${BuildDepends_BINARY_DIR}/Project/foo.cxx