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.