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/multi_files.cpp

@@ -0,0 +1,10 @@
+
+#include "multi_file_compiler_detection.h"
+
+#define PREFIX MULTI
+#include "compile_tests.h"
+
+int main()
+{
+  return 0;
+}