Improve 'configure --help' output for crypto modules

[Bruno Haible]

I wrote:
> And I got confused by
> ...
> 2) the 'configure --help' output which suggest to use an option
>    '--with-linux-crypto' — but that option does not exist.

This patch improves several things regarding the 'configure --help' output.


2021-09-19  Bruno Haible  <bruno@clisp.org>

        Improve 'configure --help' output for crypto modules.
        * m4/af_alg.m4 (gl_AF_ALG): List the hash functions for which we use the
        Linux crypto. Also mention that we use it on files only.
        * m4/gl-openssl.m4 (gl_CRYPTO_CHECK): Use manual line breaking, not the
        line-filling style of AS_HELP_STRING. Show that --with-openssl takes an
        optional argument. List the hash functions for which we use the OpenSSL
        library. Don't mention --with-linux-crypto if the af_alg module is not
        in use. Mention that --with-linux-crypto applies only to the Linux
        kernel. Mention that it gets used only when available. Mention that it
        takes precedence only for files. In the error or warning, mention the
        pre-built package name that people can install.

diff --git a/m4/af_alg.m4 b/m4/af_alg.m4
index cba09e89d..601cf6747 100644
--- a/m4/af_alg.m4
+++ b/m4/af_alg.m4
@@ -1,4 +1,4 @@
-# af_alg.m4 serial 5
+# af_alg.m4 serial 6
 dnl Copyright 2018-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -35,7 +35,8 @@ AC_DEFUN_ONCE([gl_AF_ALG],
   use_af_alg=no
   AC_ARG_WITH([linux-crypto],
     [AS_HELP_STRING([[--with-linux-crypto]],
-       [use Linux kernel cryptographic API (if available)])],
+       [use Linux kernel cryptographic API (if available) for the hash 
functions
+        MD5, SHA-1, SHA-224, SHA-256, SHA-384, SHA-512 on files])],
     [use_af_alg=$withval],
     [use_af_alg=no])
   dnl We cannot use it if it is not available.
diff --git a/m4/gl-openssl.m4 b/m4/gl-openssl.m4
index 96a7268a2..c968eb681 100644
--- a/m4/gl-openssl.m4
+++ b/m4/gl-openssl.m4
@@ -1,4 +1,4 @@
-# gl-openssl.m4 serial 5
+# gl-openssl.m4 serial 6
 dnl Copyright (C) 2013-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -19,15 +19,20 @@ AC_DEFUN([gl_CRYPTO_CHECK],
   m4_divert_once([DEFAULTS], [LIB_CRYPTO=])
 
   AC_ARG_WITH([openssl],
-    [AS_HELP_STRING([--with-openssl],
-      [use libcrypto hash routines. Valid ARGs are:
-       'yes', 'no',
-       'auto' => use if any version available,
-       'auto-gpl-compat' => use if gpl compatible version available,
-       'optional' => use if available and warn if not available;
-       default is ']gl_CRYPTO_CHECK_DEFAULT['.
-       Note also --with-linux-crypto, which will enable
-       use of kernel crypto routines, which have precedence])],
+    [[  --with-openssl[=ARG]    use libcrypto hash routines for the hash 
functions
+                          MD5, SHA-1, SHA-224, SHA-256, SHA-384, SHA-512.
+                          Valid ARGs are:
+                            'yes',
+                            'no',
+                            'auto' => use if any version available,
+                            'auto-gpl-compat' => use if GPL compatible version
+                                                 available,
+                            'optional' => use if available
+                                          and warn if not available;
+                          Default is 
']gl_CRYPTO_CHECK_DEFAULT['.]m4_ifdef([gl_AF_ALG], [
+                          Note also --with-linux-crypto, which will enable the
+                          use of Linux kernel crypto routines (if available),
+                          which has precedence for files.])],
     [],
     [with_openssl=$with_openssl_default])
 
@@ -56,10 +61,15 @@ AC_DEFUN([gl_CRYPTO_CHECK],
               [Define to 1 if libcrypto is used for $1.])])])
     fi
     if test "x$LIB_CRYPTO" = x; then
+      message='openssl development library not found for $1.
+  If you want to install it, first find the pre-built package name:
+    - On Debian and Debian-based systems: libssl-dev,
+    - On Red Hat distributions: openssl-devel.
+    - Other: https://repology.org/project/openssl/versions'
       if test "x$with_openssl" = xyes; then
-        AC_MSG_ERROR([openssl development library not found for $1])
+        AC_MSG_ERROR([$message])
       elif test "x$with_openssl" = xoptional; then
-        AC_MSG_WARN([openssl development library not found for $1])
+        AC_MSG_WARN([$message])
       fi
     fi
   fi