enhance cigloo to better handle parsing modern c and gcc extensions

[donaldsonjw]

This includes better support for gcc attributes, asm exprs/statements, and _Static_assert. Corrections to string and character literal parsing were also made.

With these changes, we can now successfully parse many current c library headers. Notcurses and GTK were used as test cases.

gcc -E -P /usr/include/notcurses/notcurses.h > notcurses.eh cigloo -gcc -macro notcurses.eh -o notcurses.sch

gcc pkg-config --cflags gtk4 -E -P /usr/include/gtk-4.0/gtk/gtk.h > gtk.eh cigloo -gcc -macro gtk.eh -o gtk.sch