eal: fix error string function

errno_autotest testcase were failed since
commit 5d7b673d5f ("mk: build with _GNU_SOURCE defined by default")
RTE>>errno_autotest
rte_strerror: 'Unknown error 11',
strerror: 'Resource temporarily unavailable'
Test Failed

There are two different version of strerror_t() based on
_GNU_SOURCE definition.

/* XSI-compliant */
int strerror_r(int errnum, char *buf, size_t buflen);

/* GNU-specific */
char *strerror_r(int errnum, char *buf, size_t buflen);

Since the GNU-specific version returns char* the exiting "if"
condition around the strerror_r fails.

Switching back to XSI-compliant version to allow

a) Portable strerror_r() usage as musl c library uses
non GNU speficic version
https://git.musl-libc.org/cgit/musl/tree/src/string/strerror_r.c

b) Based on strerror_r(3) man page, it is possible that GNU-specific
version need not use char *buf to fill error message instead it
can use the immutable static string from the library and return it.

note from strerror_r(3) man page:

The GNU-specific strerror_r() returns a pointer to a string containing
the error message.  This may be either a pointer to a string that the
function stores in buf, or a pointer to some (immutable)
static string (in which case buf is unused).

Fixes: 5d7b673d5f ("mk: build with _GNU_SOURCE defined by default")

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>

lib/librte_eal/common/eal_common_errno.c

@@ -2,6 +2,9 @@
  * Copyright(c) 2010-2014 Intel Corporation
  */
 
+/* Use XSI-compliant portable version of strerror_r() */
+#undef _GNU_SOURCE
+
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>