rocksdb: Define _GNU_SOURCE during fallocate CMake probe

The glibc headers declare fallocate only if _GNU_SOURCE is defined.
Upstream rocksdb does not need this because the probe is run with the
C++ compiler, and current g++ versions define _GNU_SOURCE
automatically.

Submitted upstream: <https://github.com/MariaDB/server/pull/2593>


diff -ur mariadb-10.5.18-downstream_modified.orig/storage/rocksdb/build_rocksdb.cmake mariadb-10.5.18-downstream_modified/storage/rocksdb/build_rocksdb.cmake
--- mariadb-10.5.18-downstream_modified.orig/storage/rocksdb/build_rocksdb.cmake	2022-11-02 13:37:22.000000000 +0100
+++ mariadb-10.5.18-downstream_modified/storage/rocksdb/build_rocksdb.cmake	2023-04-11 09:42:17.295837813 +0200
@@ -138,6 +138,7 @@
 if(WITH_FALLOCATE AND UNIX)
   include(CheckCSourceCompiles)
   CHECK_C_SOURCE_COMPILES("
+#define _GNU_SOURCE
 #include <fcntl.h>
 #include <linux/falloc.h>
 int main() {