The boehm-gc tests currently fail to build with a linker with
--no-copy-dt-needed-entries as the default. dlopen is referenced in the libgcjgc
library itself, so link it with -ldl.  The macro name EXTRA_TEST_LIBS is a bit
unfortunate now, but it is the right way to find the library name, as done for
the tests itself.

Ok for the trunk and the 4.8 branch?

  Matthias