WCDH: Make it possible to generate multiple files.

Extend the write_compiler_detection_header interface to allow
specifying a location for supplementary files, and getting the
list of resulting files as a variable.

Tests/Module/WriteCompilerDetectionHeader/compile_tests.h

@@ -0,0 +1,25 @@
+
+#define JOIN_IMPL(A, B) A ## B
+#define JOIN(A, B) JOIN_IMPL(A, B)
+
+#define CHECK(FEATURE) (JOIN(PREFIX, JOIN(_COMPILER_, FEATURE)) == JOIN(EXPECTED_COMPILER_, FEATURE))
+
+#if !CHECK(CXX_DELEGATING_CONSTRUCTORS)
+#error cxx_delegating_constructors expected availability did not match.
+#endif
+
+#if !CHECK(CXX_VARIADIC_TEMPLATES)
+#error cxx_variadic_templates expected availability did not match.
+#endif
+
+#if !CHECK(VERSION_MAJOR)
+#error Compiler major version did not match.
+#endif
+
+#if !CHECK(VERSION_MINOR)
+#error Compiler minor version did not match.
+#endif
+
+#if !CHECK(VERSION_PATCH)
+#error Compiler patch version did not match.
+#endif