build-infra: Make --enable-openjdk-only really disable custom

In the makefiles, everything closed or custom specific is guarded with ifdef OPENJDK. In configure, this isn't the case. To make the parameter --enable-openjdk-only more reliable, similar checks should be added to the closed part of configure. To enable this, the check for the parameter needs to be moved earlier in configure.ac so that the variable is available in the CUSTOM_EARLY_HOOK macro.