getrandom() is only in GLibC >= 2.2.5, and CentOS 7 uses an older
GCC version, so add CMake logic to fallback to /dev/urandom as the
Expat entroy source.
Required to address reported in-the-wild CVEs in older Expats.
This may need a tweak for Linux / BSDs to set appropriate values in
our custom expat_config.h : only macOS and Windows tested so far.
Renamed bundled Expat headers to avoid any
possibility of accidental including system headers when
using bundled Expat, or vice-versa. Should help with SVN
crashes reported by Thomas.
