Disable _FILE_OFFSET_BITS if the compiler defaults to it

Even though building glibc with LFS flags is not supported, and the
the usual way is to patch the build system to avoid it [1]; some system
do enable it by default, and it increases the requirements to build
glibc in such cases (it also does not help newcomers when trying
to build glibc).

The conform namespace and linknamespace tests also do not expect
that flag to be set by default, so disable it as well.

Checked with a build/check for major ABI and some (i386, arm,
mipsel, hppa) with a toolchain that has LFS flags by default.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=31624
Reviewed-by: DJ Delorie <dj@redhat.com>

Makeconfig

@@ -1011,6 +1011,9 @@ else
 +cflags += $(no-fortify-source)
 endif
 
+# Filter out LFS flags if compiler defines it by default.
++cflags += $(no-file-offset-bits-source)
+
 # Each sysdeps directory can contain header files that both will be
 # used to compile and will be installed.  Each can also contain an
 # include/ subdirectory, whose header files will be used to compile

configure

@@ -635,6 +635,7 @@ libc_cv_gcc_unwind_find_fde
 libc_extra_cppflags
 libc_extra_cflags
 libc_cv_cxx_thread_local
+no_file_offset_bits_source
 fortify_source
 no_fortify_source
 libc_cv_fortify_source
@@ -7784,6 +7785,37 @@ fi
 config_vars="$config_vars
 supported-fortify = $libc_cv_supported_fortify_source"
 
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main (void)
+{
+
+#ifndef _FILE_OFFSET_BITS
+# error _FILE_OFFSET_BITS not defined
+#endif
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_cpp "$LINENO"
+then :
+  libc_cv_compiler_default_file_offset_bits=yes
+else case e in #(
+  e) libc_cv_compiler_default_file_offset_bits=no ;;
+esac
+fi
+rm -f conftest.err conftest.i conftest.$ac_ext
+if test "$libc_cv_compiler_default_file_offset_bits" = yes
+then :
+  no_file_offset_bits_source="-U_FILE_OFFSET_BITS"
+fi
+
+config_vars="$config_vars
+no-file-offset-bits-source = $no_file_offset_bits_source"
+
+
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether the assembler requires one version per symbol" >&5
 printf %s "checking whether the assembler requires one version per symbol... " >&6; }
 if test ${libc_cv_symver_needs_alias+y}

configure.ac

@@ -1564,6 +1564,19 @@ AC_SUBST(no_fortify_source)
 AC_SUBST(fortify_source)
 LIBC_CONFIG_VAR([supported-fortify], [$libc_cv_supported_fortify_source])
 
+dnl Check if compiler define _FILE_OFFSET_BITS by default
+AC_PREPROC_IFELSE([AC_LANG_PROGRAM([[]], [[
+#ifndef _FILE_OFFSET_BITS
+# error _FILE_OFFSET_BITS not defined
+#endif]])],
+	       [libc_cv_compiler_default_file_offset_bits=yes],
+	       [libc_cv_compiler_default_file_offset_bits=no])
+AS_IF([test "$libc_cv_compiler_default_file_offset_bits" = yes],
+      [no_file_offset_bits_source="-U_FILE_OFFSET_BITS"])
+AC_SUBST(no_file_offset_bits_source)
+LIBC_CONFIG_VAR([no-file-offset-bits-source], [$no_file_offset_bits_source])
+
+
 dnl Starting with binutils 2.35, GAS can attach multiple symbol versions
 dnl to one symbol (PR 23840).
 AC_CACHE_CHECK(whether the assembler requires one version per symbol,

conform/Makefile

@@ -372,7 +372,7 @@ test-xfail-UNIX98/ndbm.h/conform = yes
 test-xfail-XOPEN2K/ndbm.h/conform = yes
 test-xfail-XOPEN2K8/ndbm.h/conform = yes
 
-conformtest-cc-flags = -I../include $(+sysdep-includes) $(sysincludes) -I..
+conformtest-cc-flags =  -I../include $(+sysdep-includes) $(sysincludes) -I.. $(no-file-offset-bits-source)
 # conformtest-xfail-conds may be set by a sysdeps Makefile fragment to
 # a list of conditions that are considered to be true when encountered
 # in xfail[cond]- lines in test expectations.