Clean up a bit.
Write the header only if it changes.
Merge topic 'WriteCompilerDetectionHeader-module' into next

aab05bb3 Write the header only if it changes.
4726bf3b Clean up a bit.
Remove newline.
Merge topic 'WriteCompilerDetectionHeader-module' into next

ddac2536 Remove newline.
Nullary main.
Merge topic 'WriteCompilerDetectionHeader-module' into next

e5d4049b Nullary main.
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

43106d0a Add the WriteCompilerDetectionHeader module.