MONGOCRYPT-773 check if _GNU_SOURCE is defined before defining (#977)

Avoid a possible redefinition warning when building with `_GNU_SOURCE` defined by build system.

Author: kevinAlbs

kms-message/test/test_kms_request.c

@@ -15,7 +15,9 @@
  */
 
 /* Needed for strptime */
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
+#endif
 
 #include "kms_message/kms_message.h"
 #include "kms_message_private.h"

src/mongocrypt-util.c

@@ -18,13 +18,15 @@
 #if defined(__has_include) && !(defined(_GNU_SOURCE) || defined(_DARWIN_C_SOURCE))
 #if __has_include(<features.h>)
 // We're using a glibc-compatible library
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
+#endif
 #elif __has_include(<Availability.h>)
 // We're on Apple/Darwin
 #define _DARWIN_C_SOURCE
 #endif
 #else // No __has_include
-#if __GNUC__ < 5
+#if __GNUC__ < 5 && !defined(_GNU_SOURCE)
 // Best guess on older GCC is that we are using glibc
 #define _GNU_SOURCE
 #endif

src/os_posix/os_dll.c

@@ -2,13 +2,15 @@
 #if defined(__has_include) && !(defined(_GNU_SOURCE) || defined(_DARWIN_C_SOURCE))
 #if __has_include(<features.h>)
 // We're using a glibc-compatible library
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
+#endif
 #elif __has_include(<Availability.h>)
 // We're on Apple/Darwin
 #define _DARWIN_C_SOURCE
 #endif
 #else // No __has_include
-#if __GNUC__ < 5
+#if __GNUC__ < 5 && !defined(_GNU_SOURCE)
 // Best guess on older GCC is that we are using glibc
 #define _GNU_SOURCE
 #endif