[PATCH bpf-next] tools/bpf: make libbpf _GNU_SOURCE friendly

[Yonghong Song <yhs@fb.com>]

During porting libbpf to bcc, I got some warnings like below:
  ...
  [  2%] Building C object src/cc/CMakeFiles/bpf-shared.dir/libbpf/src/libbpf.c.o
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf.c:12:0:
  warning: "_GNU_SOURCE" redefined [enabled by default]
   #define _GNU_SOURCE
  ...
  [  3%] Building C object src/cc/CMakeFiles/bpf-shared.dir/libbpf/src/libbpf_errno.c.o
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf_errno.c: In function ‘libbpf_strerror’:
  /home/yhs/work/bcc2/src/cc/libbpf/src/libbpf_errno.c:45:7:
  warning: assignment makes integer from pointer without a cast [enabled by default]
     ret = strerror_r(err, buf, size);
  ...

bcc is built with _GNU_SOURCE defined and this caused the above warning.
This patch intends to make libpf _GNU_SOURCE friendly by
  . define _GNU_SOURCE unless it is not defined
  . gnu version strerror_r has different return value than non-gnu version,
    so strerror_r return value is handled differently if _GNU_SOURCE is defined.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 tools/lib/bpf/libbpf.c       |  2 ++
 tools/lib/bpf/libbpf_errno.c | 10 ++++++++++
 2 files changed, 12 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index ed4212a4c5f9..59b748ebd15f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -9,7 +9,9 @@
  * Copyright (C) 2017 Nicira, Inc.
  */
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#endif
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdarg.h>
diff --git a/tools/lib/bpf/libbpf_errno.c b/tools/lib/bpf/libbpf_errno.c
index d83b17f8435c..286e497c50ec 100644
--- a/tools/lib/bpf/libbpf_errno.c
+++ b/tools/lib/bpf/libbpf_errno.c
@@ -40,9 +40,19 @@ int libbpf_strerror(int err, char *buf, size_t size)
 	err = err > 0 ? err : -err;
 
 	if (err < __LIBBPF_ERRNO__START) {
+#ifdef _GNU_SOURCE
+		const char *ret_buf;
+#endif
 		int ret;
 
+#ifdef _GNU_SOURCE
+		ret_buf = strerror_r(err, buf, size);
+		if (ret_buf != buf)
+			snprintf(buf, size, "%s", ret_buf);
+		ret = 0;
+#else
 		ret = strerror_r(err, buf, size);
+#endif
 		buf[size - 1] = '\0';
 		return ret;
 	}
-- 
2.17.1