Add -ftrapping-math to default cflags

This should resolve https://gitlab.freedesktop.org/pixman/pixman/-/issues/22
and make the tests pass with clang.

-ftrapping-math is already the default[1] for gcc, so this should not change
behavior when compiling with gcc.  However, clang defaults[2] to -fno-trapping-math,
so -ftrapping-math is needed to avoid floating-point expceptions when running the
combiner and stress tests.

The root causes of this issue is that that pixman-combine-float.c guards floating-point
division operations with a FLOAT_IS_ZERO check e.g.

if (FLOAT_IS_ZERO (sa))
	f = 1.0f;
else
	f = CLAMP (da / sa);

With -fno-trapping-math, the compiler assumes that division will never trap, so it may
re-order the division and the guard and execute the division first.  In most cases,
this would not be an issue, because floating-point exceptions are ignored.  However,
these tests call enable_divbyzero_exceptions() which causes the SIGFPE signal to
be sent to the program when a divide by zero exception is raised.

[1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
[2] https://clang.llvm.org/docs/UsersManual.html#controlling-floating-point-behavior

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