lib: Prevent double inclusion of config.h through system.h

lib: Prevent double inclusion of config.h through system.h

Files that include both <config.h> and "system.h" might include
config.h twice. Prevent that by adding a guard check in system.h
before including config.h.

       * lib/crc32.c: Use #ifdef HAVE_CONFIG_H instead of #if.
       * lib/error.h: Check HAVE_CONFIG_H before including config.h.
       * lib/system.h: Check both HAVE_CONFIG_H and whether
       EU_CONFIG_H is defined before including config.h.

Suggested-by: Dmitry V. Levin <ldv@strace.io>
Signed-off-by: Mark Wielaard <mark@klomp.org>