tools lib: Reinstate strlcpy() header guard with __UCLIBC__

perf tools build in recent kernels spews splat when cross compiling with uClibc:

|   CC       util/alias.o
| In file included from tools/perf/util/../ui/../util/cache.h:8:0,
|                 from tools/perf/util/../ui/helpline.h:7,
|                 from tools/perf/util/debug.h:8,
|                 from arch/../util/cpumap.h:9,
|                 from arch/../util/env.h:5,
|                 from arch/common.h:4,
|                 from arch/common.c:3:
| tools/include/linux/string.h:12:15: warning: redundant redeclaration of ‘strlcpy’ [-Wredundant-decls]
|  extern size_t strlcpy(char *dest, const char *src, size_t size);
               ^
This is after commit 61a6445 ("tools lib: Guard the strlcpy() header with
__GLIBC__").

The problem is uClibc also defines __GLIBC__ for exported headers for
applications. So add that specific check to not trip for uClibc.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petri Gynther <pgynther@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: linux-snps-arc@lists.infradead.org
Link: http://lkml.kernel.org/r/1471537703-16439-1-git-send-email-vgupta@synopsys.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/linux/string.h

@@ -8,7 +8,11 @@ void *memdup(const void *src, size_t len);
 
 int strtobool(const char *s, bool *res);
 
-#ifdef __GLIBC__
+/*
+ * glibc based builds needs the extern while uClibc doesn't.
+ * However uClibc headers also define __GLIBC__ hence the hack below
+ */
+#if defined(__GLIBC__) && !defined(__UCLIBC__)
 extern size_t strlcpy(char *dest, const char *src, size_t size);
 #endif