[foreign-jextract] RFR: configure should fail if libclang path is not specified

[Athijegannathan Sundararajan]

configure fails after the message rather than continuging after warning

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Commit messages:
 - configure should fail if libclang path is not specified

Changes: https://git.openjdk.java.net/panama-foreign/pull/190/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/190/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/190.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/190/head:pull/190

PR: https://git.openjdk.java.net/panama-foreign/pull/190