m4/pcre2_visibility.m4: add basic support for non GCC compilers

Assume that a compatible `-fvisibility=hidden` option is available
and that the compiler handles correctly `#warning` macros.

m4/pcre2_visibility.m4

@@ -4,84 +4,75 @@ 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 Originally 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 -fvisibility=hidden and the function and variable attribute
 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 MacOS 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
+dnl Set the variable VISIBILITY_CFLAGS.
 dnl Defines and sets the variable HAVE_VISIBILITY.
 
 dnl Modified to fit with PCRE build environment by Cristian Rodríguez.
 dnl Adjusted for PCRE2 by PH
+dnl Refactored to work with non GCC (but compatible) compilers
 
 AC_DEFUN([PCRE2_VISIBILITY],
 [
   AC_REQUIRE([AC_PROG_CC])
   VISIBILITY_CFLAGS=
-  VISIBILITY_CXXFLAGS=
   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 $CPPFLAGS.
-    AC_MSG_CHECKING([whether the -Werror option is usable])
-    AC_CACHE_VAL([pcre2_cv_cc_vis_werror], [
-      pcre2_save_CFLAGS="$CFLAGS"
-      CFLAGS="$CFLAGS -Werror"
-      AC_COMPILE_IFELSE(
-        [AC_LANG_PROGRAM([[]], [[]])],
-        [pcre2_cv_cc_vis_werror=yes],
-        [pcre2_cv_cc_vis_werror=no])
-      CFLAGS="$pcre2_save_CFLAGS"])
-    AC_MSG_RESULT([$pcre2_cv_cc_vis_werror])
-    dnl Now check whether visibility declarations are supported.
-    AC_MSG_CHECKING([for simple visibility declarations])
+  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 $CPPFLAGS.
+  AC_MSG_CHECKING([whether the -Werror option is usable])
+  AC_CACHE_VAL([pcre2_cv_cc_vis_werror], [
+    pcre2_save_CFLAGS="$CFLAGS"
+    CFLAGS="$CFLAGS -Werror"
+    pcre2_cv_cc_vis_werror=no
+    AC_COMPILE_IFELSE(
+      [AC_LANG_PROGRAM([[]], [[]])],
+      [
+	AC_COMPILE_IFELSE(
+	  [AC_LANG_PROGRAM([[]], [[ #warning e ]]
+	  )], [[]], [pcre2_cv_cc_vis_werror=yes]
+	)
+      ], [[]])
+    CFLAGS="$pcre2_save_CFLAGS"]
+  )
+  AC_MSG_RESULT([$pcre2_cv_cc_vis_werror])
+  if test $pcre2_cv_cc_vis_werror = yes; then
+    dnl Now check whether GCC compatible visibility declarations are supported.
+    AC_MSG_CHECKING([for GCC compatible visibility declarations])
     AC_CACHE_VAL([pcre2_cv_cc_visibility], [
       pcre2_save_CFLAGS="$CFLAGS"
-      CFLAGS="$CFLAGS -fvisibility=hidden"
+      CFLAGS="$CFLAGS -Werror -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 $pcre2_cv_cc_vis_werror = yes; then
-        CFLAGS="$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__("hidden"))) int hiddenfunc (void);
              extern __attribute__((__visibility__("default"))) int exportedfunc (void);
              void dummyfunc (void) {}
            ]],
            [[]])],
         [pcre2_cv_cc_visibility=yes],
         [pcre2_cv_cc_visibility=no])
       CFLAGS="$pcre2_save_CFLAGS"])
-    AC_MSG_RESULT([$pcre2_cv_cc_visibility])
-    if test $pcre2_cv_cc_visibility = yes; then
-      VISIBILITY_CFLAGS="-fvisibility=hidden"
-      VISIBILITY_CXXFLAGS="-fvisibility=hidden -fvisibility-inlines-hidden"
-      HAVE_VISIBILITY=1
-      AC_DEFINE(PCRE2_EXPORT, [__attribute__ ((visibility ("default")))], [to make a symbol visible])
-    else
-      AC_DEFINE(PCRE2_EXPORT, [], [to make a symbol visible])
-    fi
+      AC_MSG_RESULT([$pcre2_cv_cc_visibility])
+  fi
+  if test -n "$pcre2_cv_cc_visibility" && test $pcre2_cv_cc_visibility = yes
+  then
+    VISIBILITY_CFLAGS="-fvisibility=hidden"
+    HAVE_VISIBILITY=1
+    AC_DEFINE(PCRE2_EXPORT, [__attribute__ ((visibility ("default")))], [to make a symbol visible])
   else
     AC_DEFINE(PCRE2_EXPORT, [], [to make a symbol visible])
   fi
   AC_SUBST([VISIBILITY_CFLAGS])
-  AC_SUBST([VISIBILITY_CXXFLAGS])
   AC_SUBST([HAVE_VISIBILITY])
   AC_DEFINE_UNQUOTED([HAVE_VISIBILITY], [$HAVE_VISIBILITY],
     [Define to 1 if the compiler supports simple visibility declarations.])