RFR: 8279884: Use better file for cygwin source permission check

[Mikael Vidstedt]

For sanity, configure tries to verify that source files have the right permissions by checking the LICENSE file in TOPDIR. That file may not always be available, and for builds where it isn't configure will generate a (harmless) error along the lines of:

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Commit messages:
 - 8279884: Use better file for cygwin source permission check

Changes: https://git.openjdk.java.net/jdk/pull/7038/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7038&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279884
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7038.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7038/head:pull/7038

PR: https://git.openjdk.java.net/jdk/pull/7038