add compile time check for libgcrypt

configure.ac

@@ -33,6 +33,29 @@ AC_CHECK_HEADER(gcrypt.h, [], [AC_MSG_ERROR([unable to find the gcrypt.h header
 
 SAVE_LIBS=$LIBS
 
+# libgcrypt
+gcrypt=0
+NEED_LIBGCRYPT_API=1
+NEED_LIBGCRYPT_VERSION=1.7.0
+
+
+AM_PATH_LIBGCRYPT("$NEED_LIBGCRYPT_API:$NEED_LIBGCRYPT_VERSION", gcrypt=1)
+AC_CHECK_DECLS([gcry_mpi_set_opaque_copy], [], [], [[#include <gcrypt.h>]])
+
+if test $gcrypt = 0
+then
+  AC_MSG_ERROR([[
+***
+*** You need libgcrypt to build this program.
+**  This library is for example available at
+***   ftp://ftp.gnupg.org/gcrypt/libgcrypt/
+*** (at least version $NEED_LIBGCRYPT_VERSION (API $NEED_LIBGCRYPT_API)
+***  is required.)
+***]])
+fi
+AC_DEFINE_UNQUOTED([NEED_LIBGCRYPT_VERSION], "$NEED_LIBGCRYPT_VERSION", [required libgcrypt version])
+
+# libgnunetutil
 libgnunetutil=0
 AC_MSG_CHECKING([for libgnunetutil])
 AC_ARG_WITH(gnunet,