Check repository revision once during every compile and write revision.h only when revision has changed (fallback on .revision cache file if using tgz). Use a static sccp_revisionstr and sccp_version_str, initialized once during module load

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5206 43c1dc8c-776e-4df2-8ef1-6e829299be21
Fix check for VCS_WC_MODIFIED

git-svn-id: https://svn.code.sf.net/p/chan-sccp-b/code/trunk@5207 43c1dc8c-776e-4df2-8ef1-6e829299be21