db5.3: FTBFS with openjdk 11

[SRU Justification]
db5.3 fails to build with openjdk 11 as the default jre because:
 - debian/rules hardcodes an obsolete Java standard (1.5; 1.6 is the minimum currently supported)
 - version detection of java versions in the configure script doesn't account for the possibility of two-digit upstream version numbers.

This makes the package not currently security supportable as of 18.04 release. Since this package is seeded on the desktop image I am not looking to have this included during the freeze but am making it an SRU instead.

[Test case]
If the package builds from source and its reverse-dependencies pass their autopkgtests, this should be sufficient confirmation of a good build.

[Regression potential]
Because we are bumping the java version in this rebuild, there is a possibility that it will break compatibility with other out-of-archive java implementations that users are using. This is unlikely; 1.5 is ancient, even precise has 1.6 and trusty has 1.7 as the default. If there is concern about compatibility with precise, we could peg this to 1.6 instead of using 1.7, but there is no reasonable expectation that java bindings built on 18.04 will work on 12.04, and we know that OpenJDK 11 when it is released will break compatibility even with OpenJDK 8 in some cases which means there will definitely be jars SRUed for OpenJDK 11 that are not compatible with OpenJDK 6.