The CMakeExportBuildSettings and CMakeImportBuildSettings modules used
to export compiler paths and flags from one project and import them into
another.  The import process would force the settings on the including
project.

Forcing settings helped long ago when compiler ABIs changed frequently
but is now just a nuisance.  We've deemed the behavior harmful so this
commit simply removes it.  The modules and macros now error out if
included or called from a project that requires CMake 2.8 or higher.