Compile fix on platforms with missing pthread_rwlock_t.

Fix compilation on platforms with missing pthread_rwlock_t implementation by replacing it with pthread_mutex_t. An example of such platform can be Android OS 2.0 - 2.1, API level 5 (Eclair), Android NDK platform - android-5 where pthread_rwlock_t is not implemented and is missing in pthread.h.

In case of missing pthread_rwlock_t implementation CRYPTO_RWLOCK will work as exclusive lock in write-only mode of pthread_rwlock_t lock.

The implementation based on pthread_mutex_t must be using PTHREAD_MUTEX_RECURSIVE mode to be compatible with recursive behavior of pthread_rwlock_rdlock.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1981)

crypto/threads_pthread.c

@@ -11,8 +11,13 @@
 
 #if defined(OPENSSL_THREADS) && !defined(CRYPTO_TDEBUG) && !defined(OPENSSL_SYS_WINDOWS)
 
+#ifdef PTHREAD_RWLOCK_INITIALIZER
+    #define OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
+#endif
+
 CRYPTO_RWLOCK *CRYPTO_THREAD_lock_new(void)
 {
+#ifdef OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
     CRYPTO_RWLOCK *lock = OPENSSL_zalloc(sizeof(pthread_rwlock_t));
     if (lock == NULL)
         return NULL;
@@ -21,30 +26,62 @@ CRYPTO_RWLOCK *CRYPTO_THREAD_lock_new(void)
         OPENSSL_free(lock);
         return NULL;
     }
+#else
+    CRYPTO_RWLOCK *lock = OPENSSL_zalloc(sizeof(pthread_mutex_t));
+    if (lock == NULL)
+        return NULL;
+
+    pthread_mutexattr_t attr;
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+    
+    if (pthread_mutex_init(lock, &attr) != 0) {
+        pthread_mutexattr_destroy(&attr);
+        OPENSSL_free(lock);
+        return NULL;
+    }
+
+    pthread_mutexattr_destroy(&attr);
+#endif
 
     return lock;
 }
 
 int CRYPTO_THREAD_read_lock(CRYPTO_RWLOCK *lock)
 {
+#ifdef OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
     if (pthread_rwlock_rdlock(lock) != 0)
         return 0;
+#else
+    if (pthread_mutex_lock(lock) != 0)
+        return 0;
+#endif
 
     return 1;
 }
 
 int CRYPTO_THREAD_write_lock(CRYPTO_RWLOCK *lock)
 {
+#ifdef OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
     if (pthread_rwlock_wrlock(lock) != 0)
         return 0;
+#else
+    if (pthread_mutex_lock(lock) != 0)
+        return 0;
+#endif
 
     return 1;
 }
 
 int CRYPTO_THREAD_unlock(CRYPTO_RWLOCK *lock)
 {
+#ifdef OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
     if (pthread_rwlock_unlock(lock) != 0)
         return 0;
+#else
+    if (pthread_mutex_unlock(lock) != 0)
+        return 0;
+#endif
 
     return 1;
 }
@@ -54,7 +91,11 @@ void CRYPTO_THREAD_lock_free(CRYPTO_RWLOCK *lock)
     if (lock == NULL)
         return;
 
+#ifdef OPENSSL_THREADS_HAVE_PTHREAD_RWLOCK
     pthread_rwlock_destroy(lock);
+#else
+    pthread_mutex_destroy(lock);
+#endif
     OPENSSL_free(lock);
 
     return;