Remove the alternative symbol logic.

This should be done another way.
Merge topic 'WriteCompilerDetectionHeader-module' into next

bc4e164a Remove the alternative symbol logic.
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

55ba2326 Add the WriteCompilerDetectionHeader module.