Supplement GNUInstallDirs to support multi-arch

Debian object code libraries are in directories which include the
platform name to allow several of these to co-exist. Other distros
such as Fedora uses a simpler naming scheme (e.g. "lib64/") which
is more backward-compatible.

cmake/Modules/UseMultiArch.cmake

@@ -0,0 +1,28 @@
+# - Multiarch support in object code library directories
+#
+# This module sets the following variable
+#	CMAKE_INSTALL_LIBDIR	to lib, lib64 or lib/x86_64-linux-gnu depending
+#							the platform.
+#
+# Note that it will override the results of GNUInstallDirs if included after
+# that module.
+
+# Fedora uses lib64/ for 64-bit systems, Debian uses lib/x86_64-linux-gnu
+if ("${CMAKE_SYSTEM_NAME}" MATCHES "Linux")
+  # Debian or Ubuntu?
+  if (EXISTS "/etc/debian_version")
+	set (_libdir_def "lib/${CMAKE_LIBRARY_ARCHITECTURE}")
+  else (EXISTS "/etc/debian_version")
+	# 64-bit system?
+	if (CMAKE_SIZEOF_VOID_P EQUAL 8)
+	  set (_libdir_def "lib64")
+	else (CMAKE_SIZEOF_VOID_P EQUAL 8)
+	  set (_libdir_def "lib")
+	endif (CMAKE_SIZEOF_VOID_P EQUAL 8)
+  endif (EXISTS "/etc/debian_version")
+else ("${CMAKE_SYSTEM_NAME}" MATCHES "Linux")
+  set (_libdir_def "lib")
+endif ("${CMAKE_SYSTEM_NAME}" MATCHES "Linux")
+
+# let the user override if somewhere else is desirable
+set (CMAKE_INSTALL_LIBDIR "${_libdir_def}" CACHE PATH "Object code libraries")