only define soname-flag and version-script-flag for openjdk builds

These flags are only needed to ensure that libjvm.so can be used as a
drop-in replacement for OpenJDK's VM, and that only makes sense for
openjdk builds (without the openjdk-src option).  It didn't hurt
anything to define them unconditionally, but it was misleading.
Merge pull request #227 from dicej/flags

only define soname-flag and version-script-flag for openjdk builds