Enable more GCC warnings

This enables the following additional warnings:
-Wold-style-definition
-Warray-bounds=2 instead of the default 1
-Wformat=2, but only for GCC 4.8+ as Wno-format-nonliteral is not
 respected for older versions
-Wunused-const-variable, which enables level 2 instead of the default 1
-Warray-bounds also in debug mode through -ftree-vrp
-Wnull-dereference also in debug mode through
 -fdelete-null-pointer-checks

Closes https://github.com/curl/curl/pull/2747