Some more build fixes from OpenSUSE

These update some outdated library path searches as well as
a couple of gcc-specific improvements.

Original patch:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-compat.dif

src/lib/libast/astsa/ast.h

@@ -93,6 +93,12 @@ typedef struct
 #define pointerof(x)	((void*)((char*)0+(x)))
 #define roundof(x,y)	(((x)+(y)-1)&~((y)-1))
 
+#ifdef __GNUC__
+#if (__GNUC__ >= 4) && !defined(offsetof)
+#define offsetof(type,member)	__builtin_offsetof(type,member)
+#endif
+#endif
+
 #ifndef offsetof
 #define offsetof(type,member) ((unsigned long)&(((type*)0)->member))
 #endif

src/lib/libast/features/libpath.sh

@@ -27,7 +27,7 @@ esac
 
 ok=0
 for i in \
-	-x /lib/ld.so /lib/ld-*.so /usr/lib/ld.so /lib/rld \
+	-x /lib64/ld.so /lib/ld.so /lib64/ld-*.so /lib/ld-*.so /usr/lib/ld.so /lib/rld \
 	-f /usr/shlib/libc.so /shlib/libc.so /usr/lib/libc.so \
 	-r /usr/shlib/libc.so /shlib/libc.so
 do	case $i in

src/lib/libast/features/map.c

@@ -67,10 +67,14 @@ main()
 #define _map_malloc	1
 	printf("\n");
 	printf("#define	_map_libc	1\n");
+#endif
+#if _map_libc || defined(__linux__)
 	printf("#undef	basename\n");
 	printf("#define basename	_ast_basename\n");
 	printf("#undef	dirname\n");
 	printf("#define dirname		_ast_dirname\n");
+#endif
+#if _map_libc
 #if !_lib_eaccess
 	printf("#undef	eaccess\n");
 	printf("#define eaccess		_ast_eaccess\n");

src/lib/libast/features/sys

@@ -134,7 +134,7 @@ tst	typ_signed_size_t output{
 	}
 }end
 
-define	offsetof	(type,member)	((size_t)&(((type*)0)->member))
+define	offsetof	(type,member)	__builtin_offsetof(type,member)
 define	EXIT_FAILURE	1
 define	EXIT_SUCCESS	0
 define	MB_CUR_MAX	1