There are a large number of files in the tree that define _GNU_SOURCE
despite not actually using features hidden behind this flag. This patch
removes all these definitions in one fell swoop...
The driver vtables are read-only structures. This patch declares them as
'const' allowing the compiler to (optionally) put them in the RELRO
section. RELRO pages may be marked as read-only by the linker after
the relocations have been done ensuring that they aren't inadvertently
or maliciously altered at runtime.