✏ EDIT: visibility.m4

# visibility.m4 serial 5 (gettext-0.18.2)
dnl Copyright (C) 2005, 2008, 2010-2016 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.

dnl From Bruno Haible.

dnl Tests whether the compiler supports the command-line option
dnl -fvisibility=hidden and the function and variable attributes
dnl __attribute__((__visibility__("hidden"))) and
dnl __attribute__((__visibility__("default"))).
dnl Does *not* test for __visibility__("protected") - which has tricky
dnl semantics (see the 'vismain' test in glibc) and does not exist e.g. on
dnl Mac OS X.
dnl Does *not* test for __visibility__("internal") - which has processor
dnl dependent semantics.
dnl Does *not* test for #pragma GCC visibility push(hidden) - which is
dnl "really only recommended for legacy code".
dnl Set the variable CFLAG_VISIBILITY.
dnl Defines and sets the variable HAVE_VISIBILITY.

AC_DEFUN([gl_VISIBILITY],
[
  AC_REQUIRE([AC_PROG_CC])
  CFLAG_VISIBILITY=
  HAVE_VISIBILITY=0
  if test -n "$GCC"; then
    dnl First, check whether -Werror can be added to the command line, or
    dnl whether it leads to an error because of some other option that the
    dnl user has put into $CC $CFLAGS $CPPFLAGS.
    AC_MSG_CHECKING([whether the -Werror option is usable])
    AC_CACHE_VAL([gl_cv_cc_vis_werror], [
      gl_save_CFLAGS="$CFLAGS"
      CFLAGS="$CFLAGS -Werror"
      AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM([[]], [[]])],
        [gl_cv_cc_vis_werror=yes],
        [gl_cv_cc_vis_werror=no])
      CFLAGS="$gl_save_CFLAGS"])
    AC_MSG_RESULT([$gl_cv_cc_vis_werror])
    dnl Now check whether visibility declarations are supported.
    AC_MSG_CHECKING([for simple visibility declarations])
    AC_CACHE_VAL([gl_cv_cc_visibility], [
      gl_save_CFLAGS="$CFLAGS"
      CFLAGS="$CFLAGS -fvisibility=hidden"
      dnl We use the option -Werror and a function dummyfunc, because on some
      dnl platforms (Cygwin 1.7) the use of -fvisibility triggers a warning
      dnl "visibility attribute not supported in this configuration; ignored"
      dnl at the first function definition in every compilation unit, and we
      dnl don't want to use the option in this case.
      if test $gl_cv_cc_vis_werror = yes; then
        CFLAGS="$CFLAGS -Werror"
      fi
      AC_COMPILE_IFELSE(
        [AC_LANG_PROGRAM(
           [[extern __attribute__((__visibility__("hidden"))) int hiddenvar;
             extern __attribute__((__visibility__("default"))) int exportedvar;
             extern __attribute__((__visibility__("hidden"))) int hiddenfunc (void);
             extern __attribute__((__visibility__("default"))) int exportedfunc (void);
             void dummyfunc (void) {}
           ]],
           [[]])],
        [gl_cv_cc_visibility=yes],
        [gl_cv_cc_visibility=no])
      CFLAGS="$gl_save_CFLAGS"])
    AC_MSG_RESULT([$gl_cv_cc_visibility])
    if test $gl_cv_cc_visibility = yes; then
      CFLAG_VISIBILITY="-fvisibility=hidden"
      HAVE_VISIBILITY=1
    fi
  fi
  AC_SUBST([CFLAG_VISIBILITY])
  AC_SUBST([HAVE_VISIBILITY])
  AC_DEFINE_UNQUOTED([HAVE_VISIBILITY], [$HAVE_VISIBILITY],
    [Define to 1 or 0, depending whether the compiler supports simple visibility declarations.])
])

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