RFR: JDK-8312467: relax the builddir check in make/autoconf/basic.m4

[Matthias Baesken]

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 tests for 'if test ! -e "$OUTPUTDIR/spec.gmk"; then ...'
but the non-existance 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 .

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Commit messages:
 - JDK-8312467

Changes: https://git.openjdk.org/jdk/pull/15017/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15017&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312467
  Stats: 23 lines in 1 file changed: 2 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/15017.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15017/head:pull/15017

PR: https://git.openjdk.org/jdk/pull/15017