JAVA_SOURCES does not work with out-of-tree builds

[mbanck-cd]

If I use e.g. build-16 as build directory (as Debian package building would, in order to build jdbc_fdw for all supported Postgres versions in parallel), the build fails:

make[1]: Entering directory '/build/jdbc-fdw-hLPFKo/jdbc-fdw-0.4.0/build-16'
[...]
javac -d /usr/lib/postgresql/16/lib JDBCUtils.java JDBCDriverLoader.java JDBCConnection.java resultSetInfo.java
error: file not found: JDBCUtils.java
Usage: javac <options> <source files>
use --help for a list of possible options
make[1]: *** [/build/jdbc-fdw-hLPFKo/jdbc-fdw-0.4.0/Makefile:37: JAVAFILES] Error 2
make[1]: Leaving directory '/build/jdbc-fdw-hLPFKo/jdbc-fdw-0.4.0/build-16'