Fix "format not a string literal" errors
With GCC 6.1.

https://bugzilla.libsdl.org/show_bug.cgi?id=3375
diff --git a/src/SDL_error.c b/src/SDL_error.c
index ace5cc3..452d531 100644
--- a/src/SDL_error.c
+++ b/src/SDL_error.c
@@ -116,6 +116,8 @@
     return -1;
 }
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
 /* This function has a bit more overhead than most error functions
    so that it supports internationalization and thread-safe errors.
 */
@@ -216,6 +218,7 @@
     }
     return (errstr);
 }
+#pragma GCC diagnostic pop
 
 /* Available for backwards compatibility */
 const char *
diff --git a/src/test/SDL_test_log.c b/src/test/SDL_test_log.c
index 097372e..69696f8 100644
--- a/src/test/SDL_test_log.c
+++ b/src/test/SDL_test_log.c
@@ -50,6 +50,8 @@
  *
  * \return Ascii representation of the timestamp in localtime in the format '08/23/01 14:55:02'
  */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
 char *SDLTest_TimestampToString(const time_t timestamp)
 {
     time_t copy;
@@ -64,6 +66,7 @@
 
     return buffer;
 }
+#pragma GCC diagnostic pop
 
 /*
  * Prints given message with a timestamp in the TEST category and INFO priority.