Remove special handling of system include files (no longer needed)

The formerly used dyngen code did not work with
system include files like stdio.h.

Tests with Linux, OSX and Win32 show that this
restriction is no longer needed.

So we hopefully can remove that special piece of code.
This results in cleaner code and allows better use of
the new GCC_FMT_ATTR macro.

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>