[foreign-jextract] RFR: 8276169: --with-libclang=no jextract module is still built and jextract tool is generated

[Athijegannathan Sundararajan]

simple makefile change to exclude jextract module if --with-libclang=no is specified.

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Commit messages:
 - 8276169: --with-libclang=no jextract module is still built and jextract tool is generated

Changes: https://git.openjdk.java.net/panama-foreign/pull/607/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=607&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276169
  Stats: 5 lines in 2 files changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/607.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/607/head:pull/607

PR: https://git.openjdk.java.net/panama-foreign/pull/607