Add top-level folders to gitignore to enable out-of-source builds

.gitignore

@@ -5,20 +5,35 @@
 *.cmake
 !/cmake/*.cmake
 *~
-/test/benchmark_test
-/test/re_test
+
+# cmake files.
 /Testing
 CMakeCache.txt
 CMakeFiles/
+cmake_install.cmake
+
+# makefiles.
 Makefile
+
+# in-source build.
 bin/
-cmake_install.cmake
 lib/
+/test/benchmark_test
+/test/re_test
+
+# exuberant ctags.
 tags
+
+# YouCompleteMe configuration.
 .ycm_extra_conf.pyc
 
+# ninja generated files.
 .ninja_deps
 .ninja_log
 build.ninja
 install_manifest.txt
 rules.ninja
+
+# out-of-source build top-level folders.
+build/
+_build/