Explain zeek-config options in help output

zeek-config.in

@@ -38,11 +38,39 @@ include_dir=$(add_path "$include_dir" "@ZEEK_CONFIG_OPENSSL_INCLUDE_DIR@")
 include_dir=$(add_path "$include_dir" "@ZEEK_CONFIG_LibKrb5_INCLUDE_DIR@")
 include_dir=$(add_path "$include_dir" "@ZEEK_CONFIG_GooglePerftools_INCLUDE_DIR@")
 
-usage="\
+usage() {
-Usage: zeek-config [--version] [--build_type] [--prefix] [--btest_tools_dir] [--lib_dir] [--script_dir] [--site_dir] [--plugin_dir] [--config_dir] [--python_dir] [--include_dir] [--cmake_dir] [--zeekpath] [--zeek_dist] [--binpac_root] [--caf_root] [--broker_root]"
+    echo "Usage: zeek-config [OPTIONS]
+
+Basic options:
+
+  --build_type          Zeek build type as per cmake, lower case (e.g. 'relwithdebinfo')
+  --prefix              Toplevel Zeek distribution installation directory
+  --version             Zeek version number
+  --zeek_dist           Toplevel directory of source tree the distribution built from
+  --zeekpath            ZEEKPATH environment variable paths for this distribution
+
+Specific directories in the Zeek distribution:
+
+  --btest_tools_dir     Zeek-related BTest tooling
+  --cmake_dir           Zeek's cmake modules
+  --config_dir          Configuration files for cluster topology, zkg, etc
+  --include_dir         C/C++ header folders for Zeek and related components, colon-separated
+  --lib_dir             Toplevel folder for shared libraries, Python packages, etc
+  --plugin_dir          Native-code Zeek plugins
+  --python_dir          Python packages (Broker, ZeekControl, zkg, etc)
+  --script_dir          Toplevel folder for Zeek scripts
+  --site_dir            Site-specific Zeek scripts
+
+Toplevel installation directories for third-party components:
+
+  --binpac_root         BinPAC compiler
+  --broker_root         Broker communication framework
+  --caf_root            C++ Actor Framework
+"
+}
 
 if [ $# -eq 0 ] ; then
-      echo "${usage}" 1>&2
+      usage 1>&2
       exit 1
 fi
 
@@ -111,7 +139,7 @@ while [ $# -ne 0 ]; do
       echo $zeekpath
       ;;
     *)
-      echo "${usage}" 1>&2
+      usage 1>&2
       exit 1
       ;;
   esac