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.

3 files changed, 39 insertions, 14 deletions

diff --git a/ChangeLog b/ChangeLog
index ef588799d0..9c5d3febff 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,17 @@
+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.
+
 2021-09-18  Bruno Haible  <bruno@clisp.org>
 
 	string, wchar: Don't require undefined m4 macros (regr. today).
diff --git a/m4/af_alg.m4 b/m4/af_alg.m4
index cba09e89d6..601cf67474 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 96a7268a27..c968eb6815 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