[libpng18] use ISO standard attributes

[jbowler]

This is based on #589. It replaces the ad-hoc use of compiler specific
attributees by the ISO standard and adjusts the PNG_FUNCTION macro to
work with the ISO standard; function attributes precede function storage
classes.

• cmake: Remove architecture-specific checks for hardware optimization
• Full compile time support for target-specific code
• MIPS support clean-up
• Use ISO standard attributes

[jbowler]

Addresses #568 and obsoletes #572 by doing it in a portable way (#572 would have removed support from some compilers in favour of others, this removes some support from compilers which don't keep up to date with the standard but provides support for the current common compilers, gcc clang and vc).

Note that in the current form this completely addresses the original issue; the original /* FALLTHROUGH */ comment is retained however an extra PNG_FALLTHROUGH or (for the apps) FALLTHROUGH statement is inserted at the same place immediately before it. This matches both the ISO-C requirements and the older compiler requirements.

The change also takes advantage of the ISO spec to add what are I hope meaningful messages to the PNG_DEPRECATED usages. Adds PNG_MAYBE_UNUSED for parameters. PNG_USED is still better if it is done correctly (only on the #if path where the thing really is unused) but [[unused]] is easier, particularly where the parameter really is completely unused (C90 does not support omission of the formal parameter in the definition.)

Makes it easy to add [[sequential]] and [[reproducible]] in the future but these are not currently supported by GCC even though the GCC does support modern definitions of constexpr in C++ (i.e. it supports the [[reproducible]] variant, though [[reproducible]] does other things as well.)

[jbowler]

This is development work and depending on the choice of compiler standard to be used may be superfluous, so I've removed it until other more important issues (reported bugs) are resolved and the long term development plan is clear.