Add signed integer overflow checking to libpng.

This change moves libpng over to Clang in order to take advantage
of its stronger -ftrapv option, causing the code to abort rather
than allowing a signed integer overflow.

This change depends on Ida05e2d746a88913fc354224a7c660f7570e3adc
and I463217aef573a28069469514ad94cd85a7f38bea.

Change-Id: I03e6d217fc01ee6429a9a4d5d593379e06c4ef29
2 files changed