Performance improvement in tkOldConfig.c. See [610a73a179]. Backport … #446

Job	Run time
test (gcc)	4m 1s
build (gcc)	1m 41s
test (gcc, --disable-xft)	3m 57s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 43s
test (gcc, --enable-symbols)	3m 32s
build (gcc, --disable-xft)	1m 40s
build (gcc, --disable-xss)	1m 44s
build (gcc, --enable-symbols)	1m 18s
build (clang)	1m 54s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 58s
build (clang, --disable-xft)	2m 4s
build (clang, --disable-xss)	2m 0s
build (clang, --enable-symbols)	1m 28s
	29m 0s

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