cc/gcc: add option to use system zlib

In some cases, it might be desirable to use the system zlib

Eg. because latest gcc seem to be totally borked when it comes
to multilib, and tries to build a multilib host zlib, when it
is *absolutely* *not* needed: we want mulitlib on the target,
not on the host! Sigh... :-(

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

--- glibc-2_9.orig/nptl/init.c	2007-10-15 22:25:25.000000000 +0200

+++ glibc-2_9/nptl/init.c	2010-08-18 10:46:06.109098646 +0200

@@ -236,7 +236,7 @@

 /* This can be set by the debugger before initialization is complete.  */

-static bool __nptl_initial_report_events;

+static bool __nptl_initial_report_events __attribute_used__;

 void

 __pthread_initialize_minimal_internal (void)