relax the builddir check in make/autoconf/basic.m4

In case of issues in the configure step, we run into error messages like this:

configure: Current directory is /openjdk/jdk_4/build_mymachine.
configure: Since this is not the source root, configure will output the configuration here
configure: (as opposed to creating a configuration in <src_root>/build/<conf-name>).
configure: However, this directory is not empty. This is not allowed, since it could
configure: seriously mess up just about everything.
configure: Try 'cd /mysrcdir/jdk' and restart configure
configure: (or create a new empty directory and cd to it).
configure: error: Will not continue creating configuration in /openjdk/jdk_4/build_mymachine
configure exiting with result code 1

Probably the check could be relaxed. It currently tests for 'if test ! -e "$OUTPUTDIR/spec.gmk"; then ...' but the non-existence of spec.gmk does not mean we are not in a build directory.
We could additionally check for e.g. $OUTPUTDIR/configure-support/config.log or something related to check if it is a build directory.