Fix test.
Merge topic 'file-generate-if-different' into next

a887a7f3 Fix test.
file(GENERATE): Only write the file if content is different.

Introduce policy CMP0053 to control this behavior.
Merge topic 'file-generate-if-different' into next

1b5d016d file(GENERATE): Only write the file if content is different.
Restrict the platforms of the WriteCompilerDetectionHeader test.

This is the same condition used to do the feature tests for the
moment in GNU-CXX.cmake.
Merge topic 'WriteCompilerDetectionHeader-module' into next

bf60be87 Restrict the platforms of the WriteCompilerDetectionHeader test.
Add the WriteCompilerDetectionHeader module.

Provide a function to write a portable header to detect compiler
features.
Merge topic 'WriteCompilerDetectionHeader-module' into next

f2dbfd5b Add the WriteCompilerDetectionHeader module.
Fix tests.
Merge topic 'file-generate-if-different' into next

892519ac Fix tests.