This is workaround for windows style path confusing scons if they appear
in CPPATH etc. Paths without prefix still happen to be valid and they
start with "/", not "c:".
(cherry-picked from commit b347bc70b1)
This is needed because the CAIRO_STATUS_INVALID_SIZE constant was added in
that version.
1.10 is old enough that this change shouldn't cause problems for anyone.
For example, Debian Wheezy (2013) has Cairo 1.12.2.