autoconf: add private config.h to clang

This already exists in the CMake build, which is part of what makes
building clang separately from llvm via cmake possible. This cleans up
that discrepancy between the build systems (and sets the groundwork
for configuring clang separately, too).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149497 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Config/config.h.in

@@ -0,0 +1,27 @@
+/* include/clang/Config/config.h.in. */
+
+#ifndef CONFIG_H
+#define CONFIG_H
+
+/* Bug report URL. */
+#undef BUG_REPORT_URL
+
+/* Relative directory for resource files */
+#undef CLANG_RESOURCE_DIR
+
+/* 32 bit multilib directory. */
+#undef CXX_INCLUDE_32BIT_DIR
+
+/* 64 bit multilib directory. */
+#undef CXX_INCLUDE_64BIT_DIR
+
+/* Arch the libstdc++ headers. */
+#undef CXX_INCLUDE_ARCH
+
+/* Directory with the libstdc++ headers. */
+#undef CXX_INCLUDE_ROOT
+
+/* Directories clang will search for headers */
+#undef C_INCLUDE_DIRS
+
+#endif