Add docs about libgcrypt to INSTALL.rst.

INSTALL.rst

@@ -11,6 +11,22 @@ resources. This sequence will install the VMOD::
   > make check		# to run unit tests in src/tests/*.vtc
   > sudo make install
 
+The configure step requires code from the Autoconf Archive collection
+of m4 macros. If you encounter difficulties in the ``configure`` step,
+make sure that you have the archive installed (typically as the
+package ``autoconf-archive`` on many distributions).
+
+To configure the build for libgcrypt, ``configure.ac`` uses the macro
+``AM_PATH_LIBGCRYPT`` from the Autoconf Archive, which in turn invokes
+the ``libgcrypt-config`` utility. The macro specifies a minimum
+library and API version number that is required, currently libgcrypt
+version 1.6.3 and API version 1. If you want to build against a
+version of libgcrypt at a non-standard location (that is, whose path
+that differs from the one described by ``libgcrypt-config``), then use
+the ``configure`` option
+``--with-libgcrypt-prefix=$LIBGCRYPT_PREFIX``, where
+``$LIBGCRYPT_PREFIX`` is the alternative path.
+
 If you have installed Varnish in a non-standard directory, call
 ``autogen.sh`` and ``configure`` with the ``PKG_CONFIG_PATH``
 environment variable pointing to the appropriate path. For example,