netresolv: enable weak symbols

It's simpler to have these around, they allow autotools to find the
functions the regular way (using AC_SEARCH_LIBS) as the autotools don't
use the include file and can't see the #define we are putting there to
redirect to the __ prefixed version of the methods.

Change-Id: I6a9743e88503fbdcba98e276375bc11f73a64411
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1889
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

src/system/libnetwork/netresolv/port_before.h

@@ -46,6 +46,8 @@
 #	define	inet_ntop		__inet_ntop
 #	define	inet_nsap_addr	__inet_nsap_addr
 #	define	inet_nsap_ntoa	__inet_nsap_ntoa
+
+#define	__weak_alias(alias, sym) __asm(".weak " #alias "\n" #alias " = " #sym);
 #endif
 
 struct group;           /* silence warning */