Replace useless uses of := by = in makefiles.

GNUmakefile.in

contrib/tsearch2/Makefile

doc/Makefile

doc/src/sgml/Makefile

src/Makefile.global.in

src/Makefile.shlib

src/backend/Makefile

src/backend/access/Makefile

src/backend/catalog/Makefile

src/backend/optimizer/Makefile

src/backend/storage/Makefile

src/backend/utils/Makefile

src/backend/utils/mb/conversion_procs/ascii_and_mic/Makefile

...ckend/utils/mb/conversion_procs/cyrillic_and_mic/Makefile

...backend/utils/mb/conversion_procs/euc_cn_and_mic/Makefile

...ackend/utils/mb/conversion_procs/euc_jp_and_sjis/Makefile

...backend/utils/mb/conversion_procs/euc_kr_and_mic/Makefile

...ackend/utils/mb/conversion_procs/euc_tw_and_big5/Makefile

...end/utils/mb/conversion_procs/latin2_and_win1250/Makefile

src/backend/utils/mb/conversion_procs/latin_and_mic/Makefile