Remove obsolete -single_module linker flag #1471

Job	Run time
build (gcc)	1m 41s
test (gcc)	4m 33s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 40s
test (gcc, --disable-xft)	4m 5s
build (gcc, --disable-xft)	1m 39s
test (gcc, --enable-symbols)	3m 29s
build (gcc, --disable-xss)	1m 40s
build (gcc, --enable-symbols)	1m 13s
build (clang)	2m 35s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 55s
build (clang, --disable-xft)	1m 50s
build (clang, --disable-xss)	2m 17s
build (clang, --enable-symbols)	1m 34s
	30m 11s

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