Drop compatibility with CMake < 2.4

Drop all behavior activated by setting CMAKE_BACKWARDS_COMPATIBILITY to
a value lower than 2.4, and generate an error when projects or the user
attempt to do so.  In the error suggest using a CMake 2.8.x release.

Teach cmake_minimum_required to warn about projects that do not require
at least CMake 2.4.  They are not supported by CMake >= 3.0.

Replace the documentation of CMAKE_BACKWARDS_COMPATIBILITY with a
reference to policy CMP0001.

Modules/Platform/Darwin-icc.cmake

@@ -60,12 +60,6 @@ if(EXISTS /Developer/SDKs/MacOSX10.4u.sdk)
     CACHE STRING "Build architectures for OSX")
 endif()
 
-
-if("${CMAKE_BACKWARDS_COMPATIBILITY}" MATCHES "^1\\.[0-6]$")
-  set(CMAKE_SHARED_MODULE_CREATE_C_FLAGS
-    "${CMAKE_SHARED_MODULE_CREATE_C_FLAGS} -flat_namespace -undefined suppress")
-endif()
-
 if(NOT XCODE)
   # Enable shared library versioning.  This flag is not actually referenced
   # but the fact that the setting exists will cause the generators to support

Modules/Platform/Darwin.cmake

@@ -223,11 +223,6 @@ if(CMAKE_OSX_DEPLOYMENT_TARGET)
   endif()
 endif()
 
-if("${CMAKE_BACKWARDS_COMPATIBILITY}" MATCHES "^1\\.[0-6]$")
-  set(CMAKE_SHARED_MODULE_CREATE_C_FLAGS
-    "${CMAKE_SHARED_MODULE_CREATE_C_FLAGS} -flat_namespace -undefined suppress")
-endif()
-
 # Enable shared library versioning.
 set(CMAKE_SHARED_LIBRARY_SONAME_C_FLAG "-install_name")