Fix pixman build with older GCC releases

The following patch fixes building pixman with older GCC releases
such as GCC 3.3 and older (OpenBSD; some older archs use GCC 3.3.6)
by changing the method of detecting the presence of __builtin_clz
to utilizing an autoconf check to determine its presence. Compilers
that pretend to be GCC, implement __builtin_clz and are already
utilizing the intrinsic include LLVM/Clang, Open64, EKOPath and
PCC.
test/trap-crasher.c: Add trapezoid that demonstrates a crash

This trapezoid causes a crash due to an underflow in the
pixman_trapezoid_valid().

Test case from Ritesh Khadgaray.
pixman_trapezoid_valid(): Fix underflow when bottom is close to MIN_INT

If t->bottom is close to MIN_INT (probably invalid value), subtracting
top can lead to underflow which causes crashes.  Attached patch will
fix the issue.

This fixes bug 67484.
Pre-release version bump to 0.31.2
Post-release version bump to 0.31.3
Merge branch 'master' of git://anongit.freedesktop.org/git/pixman
JOHNE - Bump the version number to 0.31.03