Added build/.gitkeep and added build/ to .gitignores

Let's encourage the convention of building out-of-source builds into
the build folder.

Author: Martin Preisler

.gitignore

@@ -5,10 +5,10 @@
 *~
 *.pyc
 
-# RPM build files
+# Build files
 rpmbuild
 scap-security-guide.spec
-**/build/*
+build/
 
 # Ignore dist directories but remember dist/README files
 **/dist