The `cxx.try_compile` command in this file kicks `cxx.have_devel?`
internally, which recursively calls `cxx.try_link` with a different
source code. We don't want that happen (the source code compiled in
this file must be the first one). We need to fake the system.
Look up language module with `MakeMakefile.[]`, insted of a
accessing constant under that module directly, to get rid of
expose the constant to the toplevel inadvertently.
This is a test extension so we basically want test failures rather
than a configure breakage but if there is no C++ compiler, we need
no test at all because there will be no chance for the tested
header file to be used later.
This makes it possible to build the ruby binary without any C++
compiler installed in a build environment.
