Makefile:
ifneq ($(wildcard .svn),)
  OPENPBXVERSIONNUM=999999
  ifneq ($(wildcard .svn/entries),)
    OPENPBXVERSION=$(shell grep revision .svn/entries | head -n 1 | sed 's:^.*"\([0-9]\+\)".*:\1:')
  else
    OPENPBXVERSION=SVN TRUNK
  endif
else
  OPENPBXVERSIONNUM=000000
endif

result:
/*
 * version.h
 * Automatically generated
 */
#define OPENPBX_VERSION "270"
#define OPENPBX_VERSION_NUM 999999