fix GNU strerror_r() detection, the bug has been inroduced in r2600
diff --git a/auto/os/linux b/auto/os/linux index c19f3f4..0bd0488 100644 --- a/auto/os/linux +++ b/auto/os/linux
@@ -140,4 +140,4 @@ ngx_include="sys/vfs.h"; . auto/include -CC_AUX_FLAGS=$cc_aux_flags +CC_AUX_FLAGS="$cc_aux_flags -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64"