Problem building JDBC in PostgreSQL 7.2devel

After much fun building ant (which, at the very least proved I have a
working javac AND that I don't need JAVA_HOME set, I get this failure
building PostgreSQL 7.2devel with Java support:

configure:1499: checking for executable suffix
configure:1509: gcc -o conftest -O2 -g conftest.c 1>&5
configure:1530: checking how to run the C preprocessor
configure:1551: gcc -E conftest.c >/dev/null 2>conftest.out
configure:1611: checking whether gcc needs -traditional
configure:1757: checking whether to build with Tcl
configure:1781: checking whether to build with Tk
configure:1843: checking whether to build with PL/Tcl unknown support
configure:1877: checking whether to build Perl modules
configure:1904: checking whether to build Python modules
configure:1930: checking whether to build Java/JDBC tools
configure:1942: checking for jakarta-ant
configure:1942: checking for ant
configure:1979: checking whether /usr/bin/ant works
configure:2002: /usr/bin/ant -buildfile conftest.xml 1>&2
Buildfile: conftest.xml

conftest:
[javac] Modern compiler is not available - using classic compiler
[javac] Compiling 1 source file

BUILD FAILED

/tmp/pgsql/conftest.xml:3: Cannot use classic compiler, as it is not
available A common solution is to set the environment variable JAVA_HOME
to your jdk directory.

Total time: 1 second
configure: failed java program was:
public class conftest {
int testmethod(int a, int b) {
return a + b;
}
}
configure: failed build file was:
<project name="conftest" default="conftest">
<target name="conftest">
<javac srcdir="." includes="conftest.java">
</javac>
</target>
</project>

The package builds, installs and runs fine (at least to the point of
initialising the database and creating a new user) without java.

JDBC is essential to the testing I'm doing.