Extend the write_compiler_detection_header interface to allow
specifying a location for supplementary files, and getting the
list of resulting files as a variable.
Provide a function to write a portable header to detect compiler
features. Generate a preprocessor #error for unknown compilers
and compiler versions whose features are not yet recorded. This
error condition might be relaxed in the future, but for now it
is useful for verification of expectations.
Stephen Kelly