Export: Use the CMAKE_DEVEL_VERSION macro for build-export files.

Move the macro definition to the cmExportBuildFileGenerator.h header
to share it.
Export: Issue runtime error if DEVEL_CMAKE_VERSION requests old version.

While developing version 3.1.0, this macro is used with

 DEVEL_CMAKE_VERSION(3, 1, 0)

and it expands to a datestamped version such as "3.0.x.2014yyyy". When
the version of cmake is bumped to 3.1.0, the macro expands to "3.1.0".
The macro should be replaced with the static string "3.1.0" soon after
the 3.1.0 release.

Create a runtime error if the version of cmake is bumped to an encoded
version greater than 3.1.0 without first replacing the macro with a
static string.  It is not possible/practical to create a compile-time
error in this case, depending on the values of the macro parameters.
Merge topic 'cmake-devel-version-macro' into next

2197bcde Export: Issue runtime error if DEVEL_CMAKE_VERSION requests old version.
6ca6613a Export: Use the CMAKE_DEVEL_VERSION macro for build-export files.