Features: Record the oldest supported compiler.

This may be used to ensure that features are reported correctly
only if the compiler minimum version is met.
Add the WriteCompilerDetectionHeader module.

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.
Merge topic 'WriteCompilerDetectionHeader-module' into next

cb39dc60 Add the WriteCompilerDetectionHeader module.
f782417b Features: Record the oldest supported compiler.