configure.ac: Add --with-crypto= instead of many different --with-$backend

The new --with-crypto option replaces the previous backend-specific
--with-{openssl,libgcrypt,mbedtls,wincng} options and fixes some issues.

* libgcrypt or mbedtls would previously be used whenever found, even
  if configure was passed --without-libgcrypt or --without-mbedtls.

* If --with-$backend was specified then configure would not fail even
  if that library could not be found, and would instead use whichever
  crypto library was found first.

The new option defaults to `auto`, which makes configure check for all
supported crypto libraries in turn, choosing the first one found, or
exiting with an error if none can be found.

configure.ac

@@ -83,78 +83,81 @@ AC_C_BIGENDIAN
 dnl check for how to do large files
 AC_SYS_LARGEFILE
 
-found_crypto=none
-
 # Configure parameters
-AC_ARG_WITH(openssl,
-  AC_HELP_STRING([--with-openssl],[Use OpenSSL for crypto]),
-  use_openssl=$withval,use_openssl=auto)
-AC_ARG_WITH(libgcrypt,
-  AC_HELP_STRING([--with-libgcrypt],[Use libgcrypt for crypto]),
-  [ use_libgcrypt=$withval
-    LIBSSH2_CHECKFOR_GCRYPT
-  ], use_libgcrypt=auto)
-AC_ARG_WITH(wincng,
-  AC_HELP_STRING([--with-wincng],[Use Windows CNG for crypto]),
-  [ use_wincng=$withval
-    LIBSSH2_CHECKFOR_WINCNG
-  ] ,use_wincng=auto)
-AC_ARG_WITH([mbedtls],
-  AC_HELP_STRING([--with-mbedtls],[Use mbedTLS for crypto]),
-  [ use_mbedtls=$withval
-    LIBSSH2_CHECKFOR_MBEDTLS
-  ], use_mbedtls=auto
-)
-AC_ARG_WITH(libz,
-  AC_HELP_STRING([--with-libz],[Use zlib for compression]),
-  use_libz=$withval,use_libz=auto)
 
+
+# Crypto backends
+
+found_crypto=none
+found_crypto_str=""
 support_clear_memory=no
+crypto_errors=""
 
-# Look for OpenSSL
-if test "$found_crypto" = "none" && test "$use_openssl" != "no"; then
-  AC_LIB_HAVE_LINKFLAGS([ssl], [crypto], [#include <openssl/ssl.h>])
-fi
-if test "$ac_cv_libssl" = "yes"; then
-  AC_DEFINE(LIBSSH2_OPENSSL, 1, [Use OpenSSL])
-  LIBSREQUIRED=libssl,libcrypto
+m4_set_add([crypto_backends], [openssl])
+m4_set_add([crypto_backends], [libgcrypt])
+m4_set_add([crypto_backends], [mbedtls])
+m4_set_add([crypto_backends], [wincng])
 
-  # Not all OpenSSL have AES-CTR functions.
-  save_LIBS="$LIBS"
-  LIBS="$LIBS $LIBSSL"
-  AC_CHECK_FUNCS(EVP_aes_128_ctr)
-  LIBS="$save_LIBS"
+AC_ARG_WITH([crypto],
+  AC_HELP_STRING([--with-crypto=auto|]m4_set_contents([crypto_backends], [|]),
+    [Select crypto backend (default: auto)]),
+  use_crypto=$withval,
+  use_crypto=auto
+)
 
-  found_crypto="OpenSSL (AES-CTR: ${ac_cv_func_EVP_aes_128_ctr:-N/A})"
-fi
+case "${use_crypto}" in
+  auto|m4_set_contents([crypto_backends], [|]))
+    m4_set_map([crypto_backends], [LIBSSH2_CHECK_CRYPTO])
+    ;;
+  yes|"")
+    crypto_errors="No crypto backend specified!"
+    ;;
+  *)
+    crypto_errors="Unknown crypto backend '${use_crypto}' specified!"
+    ;;
+esac
 
-AM_CONDITIONAL(OPENSSL, test "$ac_cv_libssl" = "yes")
-AM_CONDITIONAL(WINCNG, test "$ac_cv_libbcrypt" = "yes")
-AM_CONDITIONAL(LIBGCRYPT, test "$ac_cv_libgcrypt" = "yes")
-AM_CONDITIONAL(MBEDTLS, test "$ac_cv_libmbedtls" = "yes")
-
-# Check if crypto library was found
 if test "$found_crypto" = "none"; then
-  AC_MSG_ERROR([No crypto library found!
-Try --with-libssl-prefix=PATH
- or --with-libgcrypt-prefix=PATH
- or --with-libmbedtls-prefix=PATH
- or --with-wincng on Windows\
-])
+  crypto_errors="${crypto_errors}
+Specify --with-crypto=\$backend and/or the neccessary library search prefix.
+
+Known crypto backends: auto, m4_set_contents([crypto_backends], [, ])"
+  AS_MESSAGE([ERROR: ${crypto_errors}])
+else
+  test "$found_crypto_str" = "" && found_crypto_str="$found_crypto"
 fi
 
-# Look for Libz
-if test "$use_libz" != "no"; then
+m4_set_foreach([crypto_backends], [backend],
+  [AM_CONDITIONAL(m4_toupper(backend), test "$found_crypto" = "backend")]
+)
+m4_undefine([backend])
+
+
+# libz
+
+AC_ARG_WITH([libz],
+  AC_HELP_STRING([--with-libz],[Use libz for compression]),
+  use_libz=$withval,
+  use_libz=auto)
+
+found_libz=no
+libz_errors=""
+
+if test "$use_libz" != no; then
   AC_LIB_HAVE_LINKFLAGS([z], [], [#include <zlib.h>])
   if test "$ac_cv_libz" != yes; then
-    AC_MSG_NOTICE([Cannot find zlib, disabling compression])
-    AC_MSG_NOTICE([Try --with-libz-prefix=PATH if you know you have it])
+    if test "$use_libz" = auto; then
+      AC_MSG_NOTICE([Cannot find libz, disabling compression])
+      found_libz="disabled; no libz found"
+    else
+      libz_errors="No libz found!
+Try --with-libz-prefix=PATH if you know that you have it."
+      AS_MESSAGE([ERROR: $libz_errors])
+    fi
   else
     AC_DEFINE(LIBSSH2_HAVE_ZLIB, 1, [Compile in zlib support])
-    if test "${LIBSREQUIRED}" != ""; then
-      LIBSREQUIRED="${LIBSREQUIRED},"
-    fi
-    LIBSREQUIRED="${LIBSREQUIRED}zlib"
+    LIBSREQUIRED="$LIBSREQUIRED${LIBSREQUIRED:+ }zlib"
+    found_libz="yes"
   fi
 fi
 
@@ -351,6 +354,22 @@ AC_C_INLINE
 
 CURL_CHECK_NONBLOCKING_SOCKET
 
+missing_required_deps=0
+
+if test "${libz_errors}" != ""; then
+  AS_MESSAGE([ERROR: ${libz_errors}])
+  missing_required_deps=1
+fi
+
+if test "$found_crypto" = "none"; then
+  AS_MESSAGE([ERROR: ${crypto_errors}])
+  missing_required_deps=1
+fi
+
+if test $missing_required_deps = 1; then
+  AC_MSG_ERROR([Required dependencies are missing!])
+fi
+
 AC_CONFIG_FILES([Makefile
                  src/Makefile
                  tests/Makefile
@@ -367,10 +386,10 @@ AC_MSG_NOTICE([summary of build options:
   Compiler:         ${CC}
   Compiler flags:   ${CFLAGS}
   Library types:    Shared=${enable_shared}, Static=${enable_static}
-  Crypto library:   ${found_crypto}
+  Crypto library:   ${found_crypto_str}
   Clear memory:     $enable_clear_memory
   Debug build:      $enable_debug
   Build examples:   $build_examples
   Path to sshd:     $ac_cv_path_SSHD (only for self-tests)
-  zlib compression: $ac_cv_libz
+  zlib compression: ${found_libz}
 ])