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.

1 files changed, 3 insertions, 9 deletions

diff --git a/Source/cmPolicies.cxx b/Source/cmPolicies.cxx
index c05de17..ab822d3 100644
--- a/Source/cmPolicies.cxx
+++ b/Source/cmPolicies.cxx
@@ -357,15 +357,9 @@ bool cmPolicies::ApplyPolicyVersion(cmMakefile *mf,
   if (majorVer < 2 || (majorVer == 2 && minorVer < 4))
     {
     mf->IssueMessage(cmake::FATAL_ERROR,
-      "An attempt was made to set the policy version of CMake to something "
-      "earlier than \"2.4\".  "
-      "In CMake 2.4 and below backwards compatibility was handled with the "
-      "CMAKE_BACKWARDS_COMPATIBILITY variable.  "
-      "In order to get compatibility features supporting versions earlier "
-      "than 2.4 set policy CMP0001 to OLD to tell CMake to check the "
-      "CMAKE_BACKWARDS_COMPATIBILITY variable.  "
-      "One way to do this is to set the policy version to 2.4 exactly."
-      );
+      "Compatibility with CMake < 2.4 is not supported by CMake >= 3.0.  "
+      "For compatibility with older versions please use any CMake 2.8.x "
+      "release or lower.");
     return false;
     }