Fix bootstrap failure for bare metal due to autoconf link tests

The AC_CHECK_FUNCS tests cause the build to fail for bare metal cross
compilers, where link tests are not allowed. Replace them with
GCC_TRY_COMPILE_OR_LINK tests instead. Skip all the Filesystem
dependency checks if not building the filesystem library.

	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Only check when
	enable_libstdcxx_filesystem_ts = yes. Check for link, readlink and
	symlink.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Remove AC_CHECK_FUNCS for link, readlink and symlink.

From-SVN: r261704

libstdc++-v3/ChangeLog

 2018-06-18  Jonathan Wakely  <jwakely@redhat.com>
 
+	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Only check when
+	enable_libstdcxx_filesystem_ts = yes. Check for link, readlink and
+	symlink.
+	* config.h.in: Regenerate.
+	* configure: Regenerate.
+	* configure.ac: Remove AC_CHECK_FUNCS for link, readlink and symlink.
+
 	LWG 3035. std::allocator's constructors should be constexpr
 	* include/bits/allocator.h (allocator): Add constexpr to constructors
 	for C++2a. Replace dynamic exception specifications with NOTHROW

libstdc++-v3/config.h.in

@@ -264,7 +264,7 @@
 /* Only used in build directory testsuite_hooks.h. */
 #undef HAVE_LIMIT_VMEM
 
-/* Define to 1 if you have the `link' function. */
+/* Define if link is available in <unistd.h>. */
 #undef HAVE_LINK
 
 /* Define if futex syscall is available. */
@@ -342,7 +342,7 @@
 /* Define to 1 if you have the `quick_exit' function. */
 #undef HAVE_QUICK_EXIT
 
-/* Define to 1 if you have the `readlink' function. */
+/* Define if readlink is available in <unistd.h>. */
 #undef HAVE_READLINK
 
 /* Define to 1 if you have the `setenv' function. */
@@ -414,7 +414,7 @@
 /* Define if strxfrm_l is available in <string.h>. */
 #undef HAVE_STRXFRM_L
 
-/* Define to 1 if you have the `symlink' function. */
+/* Define if symlink is available in <unistd.h>. */
 #undef HAVE_SYMLINK
 
 /* Define to 1 if the target runtime linker supports binding the same symbol

libstdc++-v3/configure.ac

@@ -420,7 +420,6 @@ GLIBCXX_CHECK_GTHREADS
 
 # For Filesystem TS.
 AC_CHECK_HEADERS([fcntl.h dirent.h sys/statvfs.h utime.h])
-AC_CHECK_FUNCS(link readlink symlink)
 GLIBCXX_ENABLE_FILESYSTEM_TS
 GLIBCXX_CHECK_FILESYSTEM_DEPS