Schedule shared Unicode data downloads before recursive makes

[hengweiyv]

Parallel top-level builds launch separate recursive makes for Lowercase.txt and Uppercase.txt. Both can see DerivedCoreProperties.txt as missing and download it into the same file concurrently.

Declare the shared download as a top-level prerequisite of both casing targets and the data generator, so it completes once before those recursive makes start. Add an offline regression to make check that invokes real make -j8 with stand-ins for the downloader and Julia, verifies one download, and checks that both outputs are generated. The unchanged Makefile fails this test with two downloads.

Fixes #234.

Validation on Linux:

• make test_parallel_make and make -j8 test_parallel_make pass.
• make -j8 all builds the static and shared libraries.
• charwidth, misc, valid, iterate, case, custom, and maxdecomposition tests pass.
• Full make check was not run locally because Julia is not installed.

[eschnett]

Your correction is valid.

I think the new test is overkill. This is a new test that confirms that other tests do not waste work. I would omit it.

We could simplify this even more and use a single make target to download all unicode files instead of splitting this across many make targets. I contributed that code, and in hindsight, I may have gone over the top with the makefile bits for this.

I'm waiting for someone else to comment as well.