PostgreSQL 18 support #543

[INFO] Using compiling/linking rules for Linux
/Projects/repo/pgrpms/rpm/redhat/main/non-common/pljava/main/pljava_18-1.6.9-build/pljava-1_6_9/pljava-so/src/main/c/JNICalls.c:25:9: warning: no previous declaration for ‘jniEnv’ [-Wmissing-variable-declarations]
   25 | JNIEnv* jniEnv;
      |         ^~~~~~
/Projects/repo/pgrpms/rpm/redhat/main/non-common/pljava/main/pljava_18-1.6.9-build/pljava-1_6_9/pljava-so/src/main/c/VarlenaWrapper.c: In function ‘pljava_VarlenaWrapper_Input’:
/Projects/repo/pgrpms/rpm/redhat/main/non-common/pljava/main/pljava_18-1.6.9-build/pljava-1_6_9/pljava-so/src/main/c/VarlenaWrapper.c:148:23: error: implicit declaration of function ‘GetOldestSnapshot’; did you mean ‘GetLatestSnapshot’? [-Wimplicit-function-declaration]
  148 |                 pin = GetOldestSnapshot();
      |                       ^~~~~~~~~~~~~~~~~
      |                       GetLatestSnapshot
/Projects/repo/pgrpms/rpm/redhat/main/non-common/pljava/main/pljava_18-1.6.9-build/pljava-1_6_9/pljava-so/src/main/c/VarlenaWrapper.c:148:21: error: assignment to ‘Snapshot’ {aka ‘struct SnapshotData *’} from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
  148 |                 pin = GetOldestSnapshot();
      |                     ^