Fix hidden visibility compiler test

Warnings from configure tests aren't usually checked.  gcc's
"visibility attribute not supported in this configuration" warning is
enabled by default so we don't need to add any warning flag except
-Werror.

	* configure.ac (bfd_cv_hidden): Run test with -Werror in CFLAGS.
	* configure: Regenerate.

bfd/ChangeLog

@@ -1,3 +1,8 @@
+2018-05-25  Alan Modra  <amodra@gmail.com>
+
+	* configure.ac (bfd_cv_hidden): Run test with -Werror in CFLAGS.
+	* configure: Regenerate.
+
 2018-05-23  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* elf64-ppc.c (ppc64_elf_write_core_note): Add ATTRIBUTE_NONSTRING

bfd/configure

@@ -14043,6 +14043,8 @@ fi
 
 
 
+save_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking compiler support for hidden visibility" >&5
 $as_echo_n "checking compiler support for hidden visibility... " >&6; }
 if test "${bfd_cv_hidden+set}" = set; then :
@@ -14070,6 +14072,7 @@ rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
 fi
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $bfd_cv_hidden" >&5
 $as_echo "$bfd_cv_hidden" >&6; }
+CFLAGS="$save_CFLAGS"
 if test $bfd_cv_hidden = yes; then
 
 $as_echo "#define HAVE_HIDDEN 1" >>confdefs.h

bfd/configure.ac

@@ -256,12 +256,15 @@ AC_CHECK_DECLS(strnlen)
 # This is used only by compress.c.
 AM_ZLIB
 
+save_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
 AC_CACHE_CHECK([compiler support for hidden visibility], bfd_cv_hidden,
 [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 const char hw[] __attribute__ ((__visibility__ ("hidden"))) = "Hello, World\n";
 extern void print (const char *) __attribute__ ((__visibility__ ("hidden")));]],
 	[[print (hw);]])],
   [bfd_cv_hidden=yes], [bfd_cv_hidden=no])])
+CFLAGS="$save_CFLAGS"
 if test $bfd_cv_hidden = yes; then
   AC_DEFINE(HAVE_HIDDEN, 1,
 	    [Define if your compiler supports hidden visibility.])