Avoid duplicate declarations of bsdindent's global variables.

Avoid duplicate declarations of bsdindent's global variables.

Arrange for all the variable declarations in indent_globs.h to look
like "extern" declarations to every .c file except indent.c.
This prevents linker failure due to duplicated global variables when
the code is built with -fno-common, which is soon to be gcc's default.

The method of temporarily #define'ing "extern" to empty is a hack,
no doubt, but it avoids requiring a duplicate set of variable
definitions, so it seemed like the best way.

Discussion: https://postgr.es/m/20200629165051.xlfqhstajf6ynxyv@alap3.anarazel.de