limit the feature macro stuff to __linux__

These definitions are to get access to getaddrinfo() and gmtime_r()
when using glibc. This in turn conflicts with other places (which
would have these things in their libc anyway) where using these
feature flags turns off C11 functionality we would like to use.

Bug:490

Change-Id: I66fdb7292cda788df19508d99e7303ed0d4f4bdd
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/52545
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/asn1/time_support.c

@@ -55,7 +55,7 @@
  * (eay@cryptsoft.com).  This product includes software written by Tim
  * Hudson (tjh@cryptsoft.com). */
 
-#if !defined(_POSIX_C_SOURCE)
+#if defined(__linux__) && !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 201410L  /* for gmtime_r */
 #endif
 

crypto/bio/socket_helper.c

@@ -12,8 +12,10 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#if defined(__linux__)
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #include <openssl/bio.h>
 #include <openssl/err.h>