Makefile is not parallel-safe

[ginggs]

Building utf8proc using parallel make (-j) often fails.

I believe the problem is the top-level Makefile, where there are multiple invocations of make in data/ that do know know about each other but manipulate the same files.

utf8proc/Makefile

Lines 126 to 136 in 2484e2e

	data/NormalizationTest.txt:
	$(MAKE) -C data NormalizationTest.txt
	data/GraphemeBreakTest.txt:
	$(MAKE) -C data GraphemeBreakTest.txt
	data/Lowercase.txt:
	$(MAKE) -C data Lowercase.txt
	data/Uppercase.txt:
	$(MAKE) -C data Uppercase.txt