configure.ac: Rename build/ to shake-build/

Last naming change... I promise.

configure.ac

@@ -1099,8 +1099,8 @@ if grep '	' compiler/ghc.cabal.in 2>&1 >/dev/null; then
 fi
 
 # Create the configuration for the Shake-based build system if it is present
-if test -e build/cfg/system.config.in; then
-    AC_CONFIG_FILES([build/cfg/system.config])
+if test -e shake-build/cfg/system.config.in; then
+    AC_CONFIG_FILES([shake-build/cfg/system.config])
 fi
 
 AC_CONFIG_FILES([mk/config.mk mk/install.mk mk/project.mk compiler/ghc.cabal ghc/ghc-bin.cabal utils/runghc/runghc.cabal settings docs/users_guide/ghc_config.py docs/index.html libraries/prologue.txt distrib/configure.ac])