bootstrap: Forward options after '--' to cmake

Provide an interface simpler than --init= to set cache values during
bootstrap builds.  For example:

 ./bootstrap --system-zlib -- -DZLIB_ROOT=/opt/zlib

will configure CMake with a system zlib library and initialize ZLIB_ROOT
in the cache for use by FindZLIB.

bootstrap

@@ -296,7 +296,7 @@ KWSYS_IOS_FILES="
 cmake_usage()
 {
 echo '
-Usage: '"$0"' [options]
+Usage: '"$0"' [<options>...] [-- <cmake-options>...]
 Options: [defaults in brackets after descriptions]
 Configuration:
   --help                  print this message
@@ -552,6 +552,7 @@ while test $# != 0; do
   --version) cmake_version_display ; exit 2 ;;
   --verbose) cmake_verbose=TRUE ;;
   --enable-ccache) cmake_ccache_enabled=TRUE ;;
+  --) shift; break ;;
   *) die "Unknown option: $1" ;;
   esac
   shift
@@ -1504,7 +1505,7 @@ cmake_options="-DCMAKE_BOOTSTRAP=1"
 if [ -n "${cmake_verbose}" ]; then
   cmake_options="${cmake_options} -DCMAKE_VERBOSE_MAKEFILE=1"
 fi
-"${cmake_bootstrap_dir}/cmake" "${cmake_source_dir}" "-C${cmake_bootstrap_dir}/InitialCacheFlags.cmake" "-G${cmake_bootstrap_generator}" ${cmake_options} ${cmake_bootstrap_system_libs}
+"${cmake_bootstrap_dir}/cmake" "${cmake_source_dir}" "-C${cmake_bootstrap_dir}/InitialCacheFlags.cmake" "-G${cmake_bootstrap_generator}" ${cmake_options} ${cmake_bootstrap_system_libs} "$@"
 RES=$?
 if [ "${RES}" -ne "0" ]; then
   cmake_error 11 "Problem while running initial CMake"