Allow gnulib tests to be disabled.

RWMJ:

 - Patch taken from Debian downstream package.

 - The default in the original patch was to disable the tests.  I have
   changed this to default to enable them instead.

 - Removed AC_SUBST as the variable was not being used.
java: run: Add java/.libs to LD_LIBRARY_PATH so JVM finds the right JNI file.
java: Fix bogus construction of all RStructList returned values (RHBZ#1073906).

Thanks Maarten on IRC for spotting the problem.
java: Split long lines in generated output, and add other whitespace.

No functional change.
java: Factor out common field code in RStructList.

No functional change.
java: Use correct Set*Field JNI accessors to set fields of the appropriate type.

Using the wrong accessors (somehow - I have no idea how) caused other
fields in the struct to contain incorrect values.
bindtests: Fill in all fields in dummy lvm_pv struct.

Used for testing RStruct/RStructList return values, but only
in GObject and Java bindings.
java: Add regression test for RStruct/RStructList (RHBZ#1073906).