Install headers before documentation. If documentation copying (which… #1253

Job	Run time
build (gcc)	1m 44s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 58s
build (gcc, --disable-shared)	1m 40s
build (gcc, --disable-xft)	1m 39s
build (gcc, --disable-xss)	1m 36s
build (gcc, --enable-symbols)	1m 18s
build (clang)	1m 52s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 54s
build (clang, --disable-shared)	1m 49s
build (clang, --disable-xft)	1m 53s
test (gcc)	4m 32s
build (clang, --disable-xss)	1m 59s
test (gcc, --disable-xft)	4m 9s
build (clang, --enable-symbols)	1m 27s
test (gcc, --enable-symbols)	3m 37s
	33m 7s

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