Don't bother building with Tcl 8.7 headers any more #570

Job	Run time
build (gcc)	1m 54s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 48s
build (gcc, --disable-shared)	2m 5s
test (gcc)	4m 13s
build (gcc, --disable-xft)	1m 49s
test (gcc, --disable-xft)	4m 35s
test (gcc, --enable-symbols)	3m 40s
build (gcc, --disable-xss)	1m 44s
build (gcc, --enable-symbols)	1m 36s
build (clang)	2m 16s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 55s
build (clang, --disable-shared)	1m 56s
build (clang, --disable-xft)	2m 5s
build (clang, --disable-xss)	1m 55s
build (clang, --enable-symbols)	1m 42s
	35m 13s

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