add func_attr.h to be able to use func attribs

Currently enables these for every gnu-like compilers, i.e.: gcc, clang and
icc.
use function attributes on allocation functions

This should allow gcc/clang/icc to generate better code and clarifies
function intent.
Re-integrate FEAT_SIGNS code #383

- omit FEAT_NETBEANS_INTG and FEAT_SIGN_ICONS
Re-integrate FEAT_SIGNS code #383

- omit FEAT_GUI blocks